Ant and Dec: the undisputed champions of Saturday morning kids' TV.

Can the BBC's new show wipe the silly smiles off their faces?

Yesterday, Live & Kicking kicked the bucket. For eight years it was BBC1's Saturday morning kids' show, bringing us Zoe Ball and Jamie Theakston, Emma Forbes and Andi Peters, Mr Blobby and the Leprachauns. Now, a once-proud institution has been consigned to the gunge tank of history alongside Saturday Superstore and Going Live. And it's all because of two men whose names have only six letters between them: Ant and Dec.

Anthony McPartlin and Declan Donnelly – as they aren't known anywhere except their TV programmes' end credits – met as child actors in Byker Grove, a BBC youth-club soap. After leaving that series in 1994, the two friends pinballed around the light entertainment world as actors, pop singers and presenters before they landed in ITV's Saturday morning slot in August 1998. Over on BBC1, Live & Kicking was living up to its name. Its hosts, Zoe Ball and Jamie Theakston, were credited with giving children's television some street cred and with giving hungover students of both sexes someone to lust after. A rival programme was commissioned by Children's ITV boss Nigel Pickard – about whom, more later – and after a wobbly start, SM:TV Live established its own identity and its own fanbase. When Ball and Theakston left L&K to have babies and discuss cricket respectively, the battle was over. Ant and Dec ruled Saturday mornings.

Traditionally, the BBC has beaten ITV in the time slot, and even when it has dropped behind its adversary, it hasn't dropped by much. But at SM:TV's peak, it had 2.5 million viewers to L&K's 1.3 million. The latter show had become a poisoned chalice. First one presenting team and then another was groomed for stardom only to be dumped on the scrap heap, or on Channel 5 at best.

So what has SM:TV got that L&K lost? Partly, it's what you might call the Tiswas factor: L&K was too goody-goody for its own good. Just a week ago, Auntie Beeb was interviewing two bronze-medal-holding British athletes. A channel-flick away, a silicon-enhanced WWF wrestler was plugging her pay-per-view spectacular. The BBC was sending children on an outward-bound course. SM:TV was opening with a gag about the presenters' pants. SM:TV is cheeky without trying; naughtiness comes as naturally to the show as it does to any eight-year-old. Whereas, even in its dying weeks, L&K's idea of naughtiness was to pour green liquid over its guests, a stunt which was wearing thin when Noel Edmonds did it.

On SM:TV, the games and gimmicks – Wonky Donkey, Challenge Ant, the SM:TV wave, the postbag dance – are as quick and snappy as the presenters' names: you could play a whole game of Wonky Donkey in the time it took to explain the rules of L&K's Biz Quiz. I studied the Biz Quiz intently last week, and I'm still not sure why one round required the participants to kneel on the floor and milk a cardboard cow.

SM:TV is full of ideas, L&K was full of clutter. Its production team toiled manfully with the logistics of four presenters, dozens of children, oversized props, outside broadcasts and drunkenly reeling camerawork, but they struggled in vain. Often, the series was akin to Acorn Antiques presented by the Why Don't You? gang. SM:TV cleared out the clutter to the extent that children are no more than set dressing. On L&K, the studio audience was invited to interview guests and to play school sports day games. On SM:TV, it's allowed to cheer and to answer brief questions. Otherwise, it's seen and not heard.

None of this would matter, though, if SM:TV didn't have two aces up its sleeve: Ant and Dec. No disrespect to the third presenter, Cat Deeley, the Angela Rippon to Ant and Dec's Morecambe and Wise, but there's no doubt who the stars are. Donnelly and McPartlin rank up there with Johnny Vaughan and Jonathan Ross in a select coterie of presenters who aren't officially comedians and yet are funnier than any of the nominal comics they trade gags with. They're Vic and Bob minus the misanthropy. While it's conceivable that L&K's writers might have been able to script such lines as "shut up, you foul elk" and "we're going to tread water in the sea of your letters", there's no way L&K's presenters could have delivered them. The reason why adults can be as addicted to SM:TV as children can is that the programme is almost a sketch revue – as close to Saturday Night Live as it is to Saturday Superstore. And it's all built on Ant and Dec's humour, friendship and north-eastern background.

Their enjoyment never seems fake. The received wisdom is that they aim their humour at older viewers. In fact, their secret is more or less the opposite: like Ball and Theakston on vintage L&K, they don't behave as if they're superior to their core audience of 10- to-14-year-olds. I remember when Sarah Greene would treat everyone on Saturday Superstore – including celebrity guests – as if they'd just wet themselves on their first day of primary school. Nowadays, children's TV presenters are more likely to be cool and ironic, which is just as condescending. But Ant and Dec talk to children as children talk to each other – with all the rudeness, silliness and sarcasm that entails. When a caller gets the answer wrong in Wonky Donkey, Dec is as merciless as a playground bully. When a child risks losing all their hard-won prizes in the Gamble round of Challenge Ant, Ant gives them no quarter. The child goes home with nothing, Ant gloats, and Dec invites the following week's contestant to "wipe that smirk off his face".

The BBC is hoping to do just that. L&K's replacement, The Saturday Show, begins next week, and if it doesn't have Ant and Dec, it does have the nearest thing: it was commissioned by the new-ish head of Children's BBC, Nigel Pickard. "Having commissioned them both does put me at the centre of attention," acknowledges Pickard, at themedia launch last Monday. "I'd quite like that to wear off a bit. But it's great when you have strong competition. It gives things an edge for the viewers, and for the producers. It's the viewers who benefit. The goal for us is to come up with a true alternative to SM:TV."

Pickard, a 20-year veteran of children's programming, stresses "the long-term view" – The Saturday Show could take six months or more to gather its audience. "SM:TV is very professional and well-run," he says, "but if I dug out the pilot, you'd see how much it's come on. It was all over the place."

What he's imported from SM:TV is an entertainment agenda. Saturday morning is "kids' time off", says Pickard, so there won't be any more interviews with Mo Mowlam or Richard Branson. And judging by Monday's press preview, this was a shrewd move. The Saturday Show has "confident simplicity", in the buzzwords of Annette Williams, the executive producer (an alumnus of Ball & Theakston's Live & Kicking stint). She's prioritised audience participation, bought some snazzy American cartoons and teen soaps, introduced a house band and a pair of amusing puppets, and hired two new presenters, one of whom, curiously, is Dani Behr. Can Behr really muster sincere enthusiasm when she's chatting to a giant hamster puppet? She adopted a look of faint disgust when she presided over worm-eating and granny-snogging on The Word, and she's hasn't managed to shift it.

She'll certainly guarantee The Saturday Show an FHM cover or two, but whether she'll be able to challenge Ant and Dec remains to be seen. Still, let's not forget Pickard's long-term view. One day soon Ant and Dec will want a lie-in on Saturdays, and it's a day SM:TV's producers must be dreading. "The Morecambe And Wise Show without Morecambe and Wise would be a strange thing," muses Pickard with a chuckle. "I must say, I'm glad that problem's no longer mine."

'The Saturday Show', BBC1, Saturday, 9am