'Really shocking in such a young child,' says Alice Band. 'But I suppose they pick it up at school. Where does he go, as a matter of interest?'

The lady has a blue velvet Alice band, a Laura Ashley sundress, navy blue pumps, two hot, cross-looking daughters and a massive Silver Cross pram - the sort of arrogant, outdated and unwieldy vehicle that demands a palatial hallway.

We're idling away a stifling afternoon at the Toy and Model Museum - me, Jacob, Chloe, Raphael and Jacob's friend Nicholas, six. Jacob and Nicholas are enjoying an aggressive game of shove ha'penny at a wooden table in the middle of the garden. Much shrieking and slamming of fists. Chloe and Raphael are in the queue for face-painting. I'm darting between the two parties with ice lollies and threats.

Alice Band is sitting there surrounded by her brood, close to the shove ha'penny table. I've felt her beady eyes on me once or twice. Then, "Look here," I hear her say to Nicholas. "Would you please not swear. I find it extremely offensive."

"I'm so sorry," I come over quickly, responsibly. "He's with me. What exactly did he say?"

She sighs and shoves a carton of Ribena into her toddler's hand, "Well. The 'sh' word several times and then just now the 'f' word. I'm sorry, I simply can't have my daughters hearing that kind of thing."

"No," I say. "You're absolutely right, I'm so sorry, I had no idea. He's not actually mine," (hating myself for disowning him so readily). "I mean, I haven't heard him do it."

Her dark-haired daughters - both wearing Alice bands - are staring at me, frowning into the sun. The mother strokes their heads protectively. "Well, you wouldn't," she says pityingly, "of course you wouldn't. He probably doesn't dare do it in front of you."

"Maybe not." I don't know what else to say.

She softens, puts a hand on my wrist. "Don't take this the wrong way, I can see it's not your fault."

"Oh!" I'm not having that. "No, no. He's absolutely my responsibility. I can only apologise." I turn to him, "Nicholas, I don't ever want to hear you swear when you're with me, OK?"

"Really shocking in such a young child," Alice Band begins again, apparently intent upon conversation, despite everything. "But I suppose they pick it up at school. Where does he go, as a matter of interest?" (Clearly, we can discuss him freely now that we've established he belongs to neither of us).

"Oh," I reply, rather too quickly. "A posh one actually - sorry - you know, a private one. Thomas's."

"Really?" Pure horror distorts her calm, freckled features. "In Battersea or Wandsworth?"

"Oh, I'm not sure." Frankly, they all blur into one for me.

She looks extremely troubled. "But that's where we go. Thomas's in Wandsworth."

I shrug, secretly delighted. "There you are, then. It's funny, isn't it? I mean, my kids are at the local primary school and they haven't picked up any 'swear' words there - not so far, anyway." (This is true. Please let it continue to be true. Touch wood, deep breath, fingers crossed.)

She doesn't know what to say to this. She adjusts her Alice Band and manoeuvres the fretful inhabitant of the Silver Cross out of the sun. "Well, I hope you didn't mind me saying?"

"Of course not," I say, deliriously happy. "You were absolutely right to bring it to my attention."

The little ones reach the front of the face-painting queue. You can choose from tigers or tropical island scenes or Eastern princesses, but Chloe wants her face painted like a man with a beard and moustache and Raphael just wants his black, like Batman. After a little nervous laughter, the face-painters comply. Raphael looks like he's fallen into a bag of soot. It cost pounds 2 but if he's happy, so am I.

Except that he's not. He takes one look in the mirror and hurls himself to the floor in an odious tantrum, legs flailing, untouchable.

I'm glad that Alice Band has left.

To cheer everyone up, we go and look around the shop which sells lots of expensive and unnecessary things with London buses and Teddy bears and Beefeaters on them. I firmly refuse the tourist chocolate in white paper with gold curly writing and buy everyone a rubber.

"We've already got a million rubbers," whines Jacob, and I tell him he's on the verge of being very spoilt and making me cross. "Thank you very much," says Nicholas with winning (private school) manners as he pockets his.

We also buy the story of A Fierce Bad Rabbit by Beatrix Potter because the kids are mad about her at the moment and Raphael, being a Fierce Bad Thing himself, enjoys stories about his rivals.

Back in the Museum garden, we sit on a shady bench and I read it aloud. To cut a short story even shorter, the eponymous Fierce Bad Rabbit snatches a carrot from the Good Rabbit without even asking. Fortuitously a man arrives on the scene with a gun and, believing the carrot-scoffer in question to be a bird (er, it's OK to shoot birds) fires at him on the bench. The Bad Rabbit does not die (well, how would I explain that?) but is neatly divested of tail, whiskers and carrot in the blast. It's the three-year-old Reservoir Rabbits.

Raphael adores the picture of the blast - a delicate Beatrix Potter watercolour of sudden, violent damage - and he deposits several slobbery kisses on it.

"What do you think the Bad Rabbit says when he loses his whiskers and tail?" I ask the assembled company as I shut the book.

"Bloody hell," says Chloe without a second's hesitation. And I glance up - I can't help it - just to check that Alice Band has really, truly gone.