Season's Greetings, NT Lyttelton, London<br/>King Lear, Donmar Warehouse, London<br/>Matilda, Courtyard Theatre, Stratford-upon-Avon

Will this Christmas be thoroughly merry? Not likely.

In the latest of Alan Ayckbourn's lesser comedies to vie for "forgotten gem" status, the Bunker family has gathered for Yuletide. That's four couples (middle-aged siblings and their partners) plus young children (kept offstage), Uncle Harvey (sprawled in front of the telly), and snow on its way.

A three-storey, middle-class home fills the Lyttelton stage in Marianne Elliott's new production of Season's Greetings. The outer walls are wadded with insulation, raggedly torn as if a bomb has detonated. Certainly, all isn't as cosy as it should be. Catherine Tate's Belinda may be busily draping tinsel on the tree, but her love life has become lacklustre, and her incommunicative husband Neville (Neil Stuke) routinely leaves the chores to her, and infuriatingly spends his time knocking back the booze with his old mate Eddie.

Havoc is being wrought in the kitchen by Belinda's neurotic sister-in-law, whose prissy husband (Mark Gatiss) is, meanwhile, annoying the boorish Harvey (David Troughton). This last is a potential psycho, with firearms up his sleeve. And on top of this, sexual sparks are flying between the frustrated Belinda and her sister's alluring new beau Clive (Oliver Chris).

The first half of the show is highly enjoyable, culminating in a hilariously frantic, and impressively athletic, fling between the tipsy Tate and Chris. Downstairs in the small hours, failing to be as quiet as mice, they ricochet around in a would-be carnal embrace, slamming into walls, rolling on noisy mechanical toys and waking the entire bug-eyed household.

Katherine Parkinson is wonderfully funny – with pathos too – lugging her soused deadweight of a husband up the stairs like a giant wobbling jelly. That said, the farce peaks before the interval, the upper floors of the three-storey set are redundant, and ultimately Season's Greetings feels like an under-developed, over-compressed variation on Ayckbourn's Norman Conquests trilogy, triumphantly revived two Christmases ago.

Ferociously inclement weather engulfs King Lear as the aged monarch – Derek Jacobi, with snow-white crew cut – staggers on to the storm-blasted heath. He is homeless now, his sanity ravaged by his cold-hearted daughters. This is the most electrifying scene in Michael Grandage's production at the Donmar.

The stage is plunged into near-darkness, with Jacobi reduced to a hunched shadow as lightning flares though cracks in the massive, rough-hewn rear wall. His bedraggled Fool (excellent Ron Cook) tries to call him back, but is drowned out by howling winds and thunder, startlingly intercut with still points of eerie silence, as if we're suddenly in the eye of the storm or inside Lear's head. Rather than fulminating, he whispers the speech "Blow wind ..." It's as if he believes he can cast spells, like Prospero in The Tempest, only here it's a demented regression to childish fantasies of power.

This play's theme of inverted child-parent relationships is subtly foregrounded, and Jacobi's baby-faced Lear, though regal in fur gown and coronet, is oddly infantile from the start. Holding his tummy, he sighs with satiated pleasure when Gina McKee's beady Goneril and Justine Mitchell's smiling Regan spoonfeed him with adoring speeches.

In other respects, though, this production disappoints, Jacobi somehow contriving to give a masterful performance without being deeply moving. Ian McKellen, for the RSC a few years ago, was both more heartbreaking and funnier. Indeed, the element of comedy – so intertwined in this tragedy – is almost entirely missing.

In Matilda, the girl-heroine has one quiet moment amid the mêlée where, by a strange coincidence, she sings softly of "sailing into the eye of the storm". The adorable, spindly child-actress Kerry Ingram (playing Matilda in rotation) is standing on a big pile of books and thinking. Her wizard brain allows her to escape the culture of boneheaded stupidity espoused by her unloving parents and monstrous, sport-obsessed headmistress, Miss Trunchbull. This new adaptation of Roald Dahl's book – scripted by Dennis Kelly, with songs by comedian-cum-composer Tim Minchin – is an exhilaratingly alternative family show.

Granted, the score isn't hugely original (Sondheimish here, Lloyd Webberish there), and the RSC may be preaching to the converted, but this is nonetheless extraordinary fare: a musical crusading against dumbing down. Matilda is an inspiring mini-intellectual who refuses to be crushed and gets her classmates on side – a student protest in miniature. Celebrating the ousting of Trunchbull, the final rocking number, "Revolting Children", is a jubilant riot: a classroom of small children leaping on their desks and punching the air, like the teen musical Spring Awakening but with tweenies. Choreographed by Peter Darling, Matthew Warchus's production manages to be at once boisterous, well-drilled and wittily satirical. Brilliant.

'Season's Greetings' (020-7452 3000) to 13 Mar; 'King Lear' (0844 871 7624) to 5 Feb and touring; 'Matilda' (0844 800 1110) to 30 Jan

Next Week:

Kate Bassett sees Tom Hollander in the vintage farce A Flea in Her Ear