Once again Outnumbered provides half an hour of pure happiness. The episode begins with the two brothers at loggerheads. The younger one, Ben (Daniel Roche), has changed his brother's status on Facebook to "Jake is a transvestite", so Jake (Tyger Drew-Henry) responded by changing Ben's Facebook status to "Ben died on Tuesday". Meanwhile Dad (Hugh Dennis) struggles to explain to the children what a colonoscopy is ("Will we be able to watch it live on television?"), while Mum (Claire Skinner) makes a doomed attempt to try and explain to her 13-year-old son what is meant by the objectification of women. It's consistently funny, but best of all is the adult tennis match featuring Ben as a ball-boy and seven-year-old Karen (Ramona Marquez) as the referee.

Series three of this engagingly downbeat family comedy kicks off as Pete and Sue Brockman - the two of them outnumbered by their three children - go sightseeing in London with Pete's mother. Competing tensions are as usual caught precisely as Jake, now 14, Ben, nine, and Karen, seven, each have very different ideas as to what makes for a good day out, and aren't shy of letting their parents know about it.

Hugh Dennis and Claire Skinner play Pete and Sue but their thankless roles as parents in the comedy are mirrored in real life, because they more or less have to stand back and watch the children steal the show. Much of the dialogue is improvised and Karen (Ramona Marquez, who won Best Female Comedy Newcomer at last year's British Comedy Awards) comes up with most of the best lines, including a smart run-through of the dos and don'ts of political correctness. Perhaps these are topped, though, when Ben (Daniel Roche) gives a spot-on, if scatological, analysis of Gordon Brown's political prospects.

A few of the jokes - the confusion between lesbian and Lebanon, for example - are not in their first flush of youth, and the scene in which Dennis is left to clown around on his own is jarring, but otherwise this is a note-perfect sitcom capturing the gentle mundanity of middle-class family life in Britain today.

At last - the family sitcom that reinvented family sitcoms returns for a third series. Back in 2007 Outnumbered was a blast of suburban fresh air; with its knowing kids running rings around bewildered adults, it felt dramatically and hilariously more real than anything that had gone before. It feels fractionally less fresh now but just as funny, as we rejoin the Brockmans on a sightseeing day out in London. Very soon, scarily precocious Karen (Ramona Marquez) is grumbling that she wants to go somewhere "more World- War-Two-ish" while uncontrollable Ben (Daniel Roche) is climbing on one of the Trafalgar Square lions and stabbing it with a ruler. "Die!" he shouts. "Die, Aslan, die!" Throughout, dad Pete (Hugh Dennis) wears his fixed expression of pain and confusion, like a baited bear. As a portrait of parenthood, it's terrific, even when the plotting, which includes a weary old joke involving a disabled loo, lets the side down. And the dialogue is as sharp as ever. "Look mummy," insists Karen, "I used to believe in wishes and all this nonsense but then my wish about Ben and the hyenas didn't come true."

The funniest kids on TV are back for a third series. But has Outnumbered become a victim of its own success? Back in 2007, when this was buried apologetically in the schedule's nether regions, there was a real novelty about small children being allowed to dictate proceedings. Now, after bagging three gongs at the British Comedy Awards last year, tonight's episode feels like an extended stand-up gig for Karen and Ben (Ramona Marquez and Daniel Roche).

You sit, arms folded, thinking, "Come on, then. Say something outrageous." Naturally, you don't have long to wait but the bits with the adults in between can feel like a distraction from the main event. That's not to take anything away from the show - just a sign of how hard it is to stay ahead of the game in comedy.

The Brockmans are on a sight-seeing trip around London tonight - an opportunity for Mum and Dad and Gran to be mortally embarrassed in front of a variety of internationally-recognisable landmarks. And from the one-liners the kids come out with, it really can't be much longer before they're invited on to the Have I Got News For You panel.

There are jokes about Gordon Brown tonight and the death of Diana that might have provoked howls of outrage if they were uttered by an adult. They still might but the power of these kids is that they can get away with anything - as their beleaguered parents (Hugh Dennis and Claire Skinner) know only too well.

If only because it centres so much on the precocious (yet, for the most part, just the right side of annoying) younger members of the cast, there's an obviously limited shelf to this series, centred on the chaotic everyday life of a middle-class south London family. So, who knows, this may well be both the first and last Outnumbered Christmas special.

If it is, it's comfortably up to the standard of the two full series we've enjoyed so far, as we descend upon the Brockman family - Pete (Hugh Dennis), Sue (Claire Skinner) and their unruly offspring Ben, Jake and Karen (Daniel Roche, Tyger Drew-Honey and Ramona Marquez) - on a less than blissful Boxing Day.

Parents-under-siege sitcom Outnumbered is a slow-burning hit that's steadily accumulated both favourable ratings and gongs (it picked up three British Comedy Awards earlier this month). Rightly so, because it's a rare beast: a comedy that captures the chaos of family life without lapsing into sentimentality. This festive episode, then, is a welcome taster for the third series next spring. It's Boxing Day in the Brockman household and, along with Santa, some burglars have squeezed down the chimney. As usual, precocious, pet-obsessed seven-year-old Karen (the remarkable Ramona Marquez) steals the best scenes - she's not only lost the school hamster under the floorboards, but takes it upon herself to make everybody else's New Year's Resolutions, with typical tact. Meanwhile, brother Ben (mop-topped tyke Daniel Roche) wreaks gleeful havoc with a mechanical hand and eldest Jake (the preposterously named Tyger Drew-Honey) is trying to find Awol grandfather Frank (David Ryall). The increasingly senile old goat couldn't be hiding with the hamster, could he? Parents Pete and Sue (Hugh Dennis and Claire Skinner) preside over this pandemonium with beleaguered bafflement.

In normal years, The Royle Family would be the sitcom special to be most keenly anticipated, but after last Christmas's aberration, "The New Sofa", judgement should be reserved on Caroline Aherne's latest reunion, "The Golden Egg Cup" (Christmas Day, 9pm BBC1). For unalloyed excitement, the 'Outnumbered Christmas Special' has me slathering at the chops. It's Boxing Day, and Andy Hamilton and Guy Jenkin's recognisably modern metropolitan family, the Brockmans, has been burgled - and I don't mean harassed parents Hugh Dennis and Claire Skinner getting every scene stolen from under their noses by the improvising child actors, Tyger-Drew Honey, Daniel Roche and Ramona Marquez.

Why Outnumbered's junior rebel is the ultimate kids' hero

It's just possible that the best cult TV hero for under-10s is a curly-haired kid whose work has never been shown pre-watershed, unless you count an ad for Kingsmill bread. A hugely unscientific survey of children of my acquaintance reveals that those who have seen the BBC comedy series Outnumbered, about a flustered but happy middle-class family, all want to be Ben, the seven-year-old middle son, played by Daniel Roche.

Outnumbered lost out to The IT Crowd at the Baftas this year, which wasn't just baffling it was also a real pity, because Guy Jenkin and Andy Hamilton's winning portrayal of the minor absurdities of family life, and its brilliant, scene-stealing child actors, deserve proper recognition. In a repeat of series two's first episode, the family, including hapless mum and dad Sue and Pete (Claire Skinner and Hugh Dennis), is off to a wedding. As always they hover perilously close to being late as violence-obsessed Ben (Daniel Roche) debates if hitting someone who is attacking you with a shovel would be OK, limpid-eyed Karen (Ramona Marquez) locks herself in the bathroom and Jake (Tyger Drew-Honey) worries. It's achingly funny and packed with lovely moments, including Karen's remorseless quizzing of the increasingly unnerved bride, that always end with the grown-ups being outmanoeuvered.

There's good news for fans of Outnumbered. The unconventional family sitcom - which uses some improvisation - will be returning for a second series on BBC One at the end of September.

Hugh Dennis and Claire Skinner work well together as the hard-pressed parents of three small children. The kids themselves - Jake (Tyger Drew-Honey), Ben (Daniel Roche) and Karen (Ramona Marquez) - are terrific.

Written by Guy Jenkin and Andy Hamilton (who created Drop the Dead Donkey), the improvisational sections work surprisingly well, especially the off-the-cuff lines delivered by the kids. The lines are so good that at times Dennis and Skinner have to suppress their own wry smiles. In addition, Dennis is a gifted comedian who can also improvise, so it's a winning combination all-round.

The series became quite essential viewing last September, despite the BBC's bizarre idea of stripping it in two bunches of three consecutive episodes across a fortnight. This sort of show works much better with a more conventional regular weekly spot. Let's hope the BBC gets the scheduling right for the new series.