The Lightwater Leisure Centre is a functional hangar-like brick and steel building in earshot of the M3 motorway in Surrey, and if it ever looks at its best, it's not on this dank, grey, early winter's morning. Inside there's a bustling, purposeful air, perhaps common to all fitness centres (they're not really my milieu), and stuck to the reception desk is an A4 print-out urging members to 'Vote for Dani and Vincent' – a clue to the fact that this is where Strictly Come Dancing contestant Dani Harmer and her professional ballroom-dancing partner Vincent Simone, come for seven hours a day, four days a week, perfecting the routines to be performed to the nation on Saturday night television.

And the nation is watching, by and large. This year, Strictly Come Dancing – or Strictly as everyone calls it – has pulled away from its great TV talent show rival, ITV1's The X Factor, attracting two million more viewers than the increasingly tired Simon Cowell cash cow. For the contestants, the show is an opportunity to learn the quickstep, samba or tango but also, more importantly, to increase their profile, re-invigorate flagging thespian or music careers, or help make the transition from politics/sport/journalism into the nebulous but potentially very profitable world of celebrity.

In the case of Dani Harmer, now 23 years old but for generations of CBBC viewers, the eponymous teenage protagonist of the children's TV drama The Story of Tracy Beaker and its sequel Tracy Beaker Returns, as well as the sitcom Dani's House, Strictly is a chance to ease her way from a highly successful career as a child actor, and into more adult roles.

"When I got offered the show I pretty much bit their arm off," she admits, after she is deposited by her mother, Jill, who then leaves us alone, retreating to the coffee shop to interact with her laptop. In stature, Jill is much like her 5ft 1in daughter, whose partnership with Vincent Simone has been dubbed 'Team Smurf', while chief judge, Len Goodman, calls her either "Little Miss Dynamite" or "Munchkin". "Len is the only person I would ever let call me Munchkin," says Harmer. "Anyone else would get a thump on the nose."

In the two shows before we meet, Harmer has been given four nines-out-of-10 by the judges, bringing the audience at Wembley Arena to their feet with her quickstep – it was "fast, efficient, fearless", according to the pantomime villain of the judging panel, the acerbic Craig Revel Horwood. But here, in this hangar amid the birch and spruce Surrey woodland, is where the hard graft takes place – this is the dressed-down weekday reality behind the sequinned Saturday-night glamour.

She and Vincent are here from Monday until Thursday, this week learning a Viennese waltz. "Then on Friday we go into the studio [at BBC Television Centre in west London] where we do a camera rehearsal," she says, "and then we've got costume fittings, and obviously the tan needs to be done. And then on Saturday we go through it with the band… twice, and then you get a dress rehearsal and then you get a bit of food and then you're ready to rock'n'roll on the live show. It's a pretty manic day."

Wouldn't it be easier to just sit in the Australian rainforest, eating rice and beans and being terrorised by antipodean wildlife on I'm a Celebrity… Get Me Out of Here!? That was the road taken recently by another actress who has grown up on television, Helen Flanagan, who played Rosie Webster on Coronation Street from the age of 12. "Not really, no," says Harmer. "It depends on what you're trying to gain. On Strictly we're learning something and sometimes making complete idiots out of ourselves, but enjoying ourselves. It's not your typical 'Let's sit in a house and get filmed 24 hours a day' – then they put in all the worst bits."

If you are over the age of, say, 25, there is a strong chance you may never have heard of Harmer before this year's competition, but since 2002 she has been a fixture of children's BBC television – first as novelist Jacqueline Wilson's care-home protagonist Tracy Beaker, which ran in one form or another until 2006, or in the sitcom Dani's House, in which she played an aspiring actress. "Dani's House has a special place in my heart because I helped to create it," she says, "which is amazing, for someone my age to have that amount of input into a series."

Indeed it is. Both Dani's House, which ended this summer, and Tracy Beaker, continue to be repeated on CBBC, although Harmer can't bring herself to watch the latter. "Oh, God, no. I avoid them at all costs. I don't think people want to see even pictures of themselves when they were in their teenage years, let alone on film. I had the braces and the spots and the frizzy hair – and it was all on the telly."

Her showbiz career actually began at the age of six, in the original West End production of The Who's rock opera, Tommy, playingf the four-year-old version of the Pinball Wizard – having been put up for the role by her Saturday morning drama club in Windsor. "I ended up getting it and my mum was, 'Oh my goodness, I don't know quite what's happening here'," says Harmer. "I ended up going to Roger Daltrey's parties at the age of six. I can't really remember too much but my dad was in his element."

Her mother, a former legal worker, and father, Andy, a mechanic, who live just across the M3 in Bracknell, Berkshire (as does Dani, but, er, in Dani's house) apparently aren't anything like classic stage-school parents. "Everyone in my family thinks I'm very odd for doing what I do," says Harmer. "They're 100 per cent supportive but they have normal jobs. My younger sister, Betsy, is a hairdresser, so, yeah, I'm definitely the black sheep of the family."

Indeed both Dani and her mother Jill both seem refreshingly normal; "grounded" is how Harmer describes herself and her family, and an undercurrent of dry humour speaks of an un-starry-eyed worldliness about her chosen profession ("I was in the musical Annie, obviously," she deadpans. "I'm a child star and every child star's got to be in Annie at some point").

All child actors are legally required to have a full-time chaperone, and the six-year-old Harmer's chaperone on Tommy suggested she apply for a scholarship (which she duly won) to Redroofs Theatre School in Maidenhead, whose most famous former pupil is Kate Winslet. Harmer remained there until her GCSEs (she passed only three), but it's not a time she looks back on with any fondness. It must have been easier to be surrounded by other child performers, I had suggested, than to be a target for bullies, as sometimes happens to juvenile actors at ordinary schools after they appear on television.

"I think it's pretty much worse if you go to a theatre school as far as jealousy goes," says Harmer, "because everyone there's striving for the same thing. So if you get a part there's absolutely no way you're going to get any congratulations from anyone. It was like being in Fame 24/7, it was a nightmare. I found the whole thing a bit weird because I am quite a normal person. I think you have to be a certain person to go there and enjoy it." What sort of person? "Someone who likes doing a lot of jazz hands, I guess.

"Anyway I don't think it's necessary to be trained any more. Back in the day you needed to have gone to Rada or something, but it's so not necessary now. I just think it [acting] is one of them things you can't really teach – I think you've either got it or you haven't. You can learn to sing and you can learn how to dance but I don't think you can learn how to act."

Harmer also believes it's better to learn on the job. "I probably drove everyone mad when I first started Tracy Beaker because I wanted to know every single person's job. I'd go and sit with one of the gaffers and find out about lights and camera assistants and find out what they do."

She has worked outside of children's television – playing David Hayman's daughter in four series of Lynda La Plante's Trial and Retribution ("It was quite heavy but I never read the full scripts, just my little section"), and as Nicholas Lyndhurst's daughter in After You've Gone, a prime-time sitcom written by My Family creator Fred Barron. And then there was the attempt at a recording career when Universal/Decca signed her up in 2008 on a £3 million contract. A single ("Free", the theme song to Dani's House) was released, but failed to reach the Top 100, and the subsequent album, Superheroes, was never released.

"We gave it a go, but it was a half-arsed attempt really," she says. "I never wanted to be a singer – it wasn't my dream at all but when you've got Universal Records offering you a contract you're not going to turn it down. And it was a great experience for me just to see what the music industry is like, which is even stranger than the acting world – it's just so cut-throat. If you don't get that number one single you're just dropped straightaway, but no one tells you."

So what next, once January's month-long Strictly Come Dancing tour comes to an end? "I want to do everything," she says. "British films… I want to do more drama… musicals… Doctor Who's assistant would be my dream job, but I literally want to do everything. I'm still young. It's difficult at the moment because I'm so in this bubble I haven't time to meet people or do anything but dance."

Whether or not she makes the Strictly final will have to be seen, but as I write this Harmer remains third favourite, some distance behind Denise van Outen and Olympic gymnast Louis Smith. "It's crazy – I didn't expect to have this exposure," she says. "The public really have an opinion about Strictly." And some of the public – Twitter trolls, to be precise – have had an opinion about the size-six Harmer and her figure. "People set up these accounts just to have a go at celebrities and if that's what they want to do, by all means do that," she says. "It's no skin off my nose – I know what size I am and I'm quite happy with it." Did she read the comments? "No, never. I'm a happy, positive person and I don't need other people's opinions to bring me down. I put it down to jealousy. They just want to drag you down. I won't be dragged down."

By the same token, neither is she that interested in becoming some sort of saintly role model, to judge by the way she steps outside between interview and photo-shoot for a cigarette. I like her – I like her a lot; she could go far, even further than she's been. On leaving I say goodbye to her mother. "Get what you wanted then?" says Jill in the similar, laconic but not unfriendly tones as her daughter. One last question for Dani then: does she ever feel she has missed out on a normal childhood?

"I guess I missed out on quite a lot, but at the same time I had the best childhood ever," she says. "I got to meet the Queen, I got to go to the Baftas, I watched all these great, fun things that people like me don't normally get to do, so, yeah, I missed out on a normal – whatever normal is – childhood. But I wouldn't swap mine for anybody's because it was brilliant."

'Strictly Come Dancing' continues tonight at 6.50pm on BBC1.

Generation game The highs and lows of being a child star

Nicholas Hoult

Ten years on from their double act in About a Boy, it's hard to say who is now the bigger name in Hollywood: Hugh Grant, or his young co-star Nicholas Hoult. After his debut as a world-weary 12-year-old in the likable adaptation of Nick Hornby's novel, Hoult chose his roles well and was not too proud to be chaperoned by his mother on set until the age of 16 – when he took his career into a more mature direction, playing the cocksure teenager Tony Stonem in Channel 4's Skins. The part revealed that the awkward boy of 2002's About a Boy had grown into a strikingly handsome young man. Hollywood sat up and took notice after he played the object of Colin Firth's lust in Tom Ford's A Single Man (2010) and blockbuster roles in X-Men: First Class and the upcoming Jack the Giant Slayer followed. Now 23, and dating Hunger Games star Jennifer Lawrence, he has thus far meticulously evaded even a hint of scandal.

Kristen Stewart

Although her face is now one the most recognisable in the world, Kristen Stewart owed her first big break to a passing resemblance to another former child star, Jodie Foster. The pair co-starred as mother and daughter in the 2002 thriller Panic Room when Stewart was only 11. Born and bred in Los Angeles to parents who worked in show business, Stewart says she was not groomed for stardom and her career gathered pace slowly, with films such as Speak (2004), as a teenage rape victim, and Into the Wild (2007), as an infatuated singer. By the time casting began for the film adaptations of Stephenie Meyer's phenomenally successful Twilight novels, Stewart had become Hollywood's go-to brooding teenager. She was cast as the heroine, Bella Swan, and the saga's success – coupled with Stewart's real-life romance with co-star Robert Pattinson – have made her one of the world's richest, but most fiercely scrutinised actresses. An affair with the director Rupert Sanders saw her vilified by legions of Pattinson's female fans across the globe.

Jamie Bell

You may still hear 26-year-old Jamie Bell referred to as "the Billy Elliot boy" – an example of how just one very good performance can elevate a child from total obscurity to instant renown, but leave them with an unshakeable label. Born in Billingham, County Durham, he was chosen for his most famous role from 2,000 boys. Despite the sometimes unwanted attention Billy Elliot brought him, Bell has quietly gone about building a respectable film career, catching the eye of Peter Jackson in King Kong (2005) and being cast again in the director's The Adventures of Tintin (2011). He was also nominated for Best Actor at the British Independent Film Awards for his leading role in 2007's Hallam Foe. This year he married a fellow child star, American actress Evan Rachel Wood (of Thirteen and True Blood fame), who he met while co-starring in a Green Day video in 2005.

Ariel Winter

The childhood, or lack thereof, of Ariel Winter, 14-year-old star of the hit American TV comedy Modern Family, has become a national obsession in the USA. On the screen since her debut in a whipped-cream commercial at the age of six, Winter found stardom playing the middle sibling in the dysfunctional Dunphy family. Last month she was removed from her California home following allegations of physical and emotional abuse by her mother, Crystal Workman. She was taken into the care of her 34-year-old sister Shanelle Workman – a former child actor and television star herself. Their brother, Jimmy Workman, played Pugsley Addams in the The Addams Family film in 1991, aged 11. Winter's mother denies the allegations against her. The family are now caught in a custody battle under the spotlight of the national press with allegations made in court that Winter's mother dressed her up to look older and deprives her of food.

Matthew Lewis

Of all the Harry Potter films' gaggle of child stars, few would have predicted that it would be Matthew Lewis who would emerge as the pin-up boy. He began acting at five years old and was cast as the chubby, awkward Neville Longbottom in Harry Potter and the Philosopher's Stone at the age of 11. Lewis, from Leeds, has quietly grown into an actor of stature and a heartthrob for thousands of female fans. He played the part in all eight Harry Potter films and despite enduring the ignominy of having the erratic growth spurts of his awkward teenage years played out in front of a global audience, Lewis remained sheltered from the spotlight. It was not until he arrived at the premiere for the final film in 2011, arm-in-arm with a beautiful girlfriend, that anyone noticed Neville Longbottom had grown up. He has since embarked on a blossoming stage career, making his West End debut in Jonathan Lewis' Our Boys earlier this year.

Charlie Cooper