Tony has been coming to the gym for less than two months, but he's a pro already. Barely breaking a sweat as he cranks his treadmill up to the max, he announces that he is "in training". The biggest event in his sporting calendar might be a while off, but he is taking no chances. Thanks to his thrice-weekly workout sessions, Tony is confident victory will be his at next June's sports day.

Tony is six years old. He is one of the keenest (and teeny-tiniest) converts to the UK's first gym dedicated to six to 11-year-olds. With its Playmobil-proportioned equipment and dizzying colour scheme, the Kids gym at the Magnum Leisure Centre in Irvine on Scotland's west coast claims to be leading the monumental struggle against childhood obesity. Along with its equally new facility for 11 to 16-year-olds, the centre is on a mission to encourage even the littlest of little 'uns to embrace a healthier lifestyle.

Eyebrows are often raised at the notion of encouraging already fresh-air-starved children to spend yet more time indoors (and charging them £2.50 for the privilege). For others, such facilities can't be conjured up quickly enough.

Everyone, however, agrees that something drastic has to be done to combat Scotland's catastrophically high childhood obesity problem.

Figures released by the health department of the Scottish Executive suggest that one-third of all 12-year-olds in Scotland are overweight, with a terrifying one in five deemed "clinically obese". NHS Scotland reported that Scots youth are among the fattest in the developed world - only Italy and Malta have more overweight children. What's more, recent research by the University of Edinburgh revealed that only 22.8% of girls and 38.4% of boys in Scotland meet the recommended guidelines for levels of physical activity.

From the outside, the Magnum complex looks more retro than revolutionary. Perched on the bleak harbourside, its grubby exterior has seen better days. Step inside and down the stairs and things start to look a lot snazzier. The bass of some charty house music is just audible and the pulse gets going as you turn down a fairy-lit, purple and orange painted corridor. Push through the double doors and you're in teeny/tweeny heaven - part disco, part dinky Pleasure Beach - all flashing lights and sleek black machines and hip young staff.

This headache-inducing Shangri-la is the brand new Shokks fitness centre, for 11 to 16-year-olds, one of two new ventures that are part of the Challenging Childhood Obesity project. On one side of the 300 sq metre room, in its own little brightly coloured, mirror-lined suite, is the unimaginatively monickered kids gym.

Three boys are beginning their workout with 20-year-old instructor Cameron Carmichael. There's Tony, training for sports day, and his schoolfriend Andrew, both six, and Arron, seven. Some sessions have up to 12 children, but you get the feeling that Cameron has got enough on his hands with this trio. Together they boast enough raw energy to power the whole of Ayrshire.

Peter Mackreth is a senior lecturer in physical activity and obesity management at Leeds Metropolitan University, and helps coordinate its highly publicised "fat camps". While generally in favour of increased opportunities for exercise among young people, he has reservations about the kind of facilities on offer at the Magnum: "My main concern would be to ensure that the children are properly supervised, and that all activities are age-appropriate and developmentally appropriate. The main thing is that the experience is safe and enjoyable."

Experts stress that it is particularly important that any resistance machines are set to a suitable level, as small limbs are particularly susceptible to damage and tearing. If weights are used, it is imperative that they are not too heavy, and lifted using the correct form and techniques.

The tailored package offered at the under-11s gym in Irvine is based around the principles of gymnastics, and combines equipment use and group activities such as dodgeball and hopscotch. Put together in conjunction with the Italian fitness company Panatta, the diddy equipment is specially designed with small bodies in mind and aims to promote movement, balance and coordination rather than sinister baby bodybuilding. All the machines are fitted with big belisha beacons that go off when the children aren't performing the exercises properly, and the specially trained staff are on hand at all times.

It takes a bit of getting used to, seeing solitary tots sweating on a stationary bike like adults on a pre-holiday healthkick. Nevertheless, there are welcome differences between this gym and its adult equivalent.

The only outlandish branding is on the children's outfits, and there are no idealised pictures of perfect bodies, no MTV looping videos or painfully perfect pop stars to provide skewed role models. Although there is a machine dispensing sugary juice, the children are encouraged to make regular trips to the water cooler.

Andrew Orr is busy giving his lower back a seeing-to on the Tommy machine, his rather over-enthusiastic rowing causing the lights to flash almost permanently. His mum, Lindsay, looking on through the window (it's compulsory for mums and dads of all under-eights to stay in the building), has no time for the theory that there's something a little spooky about children who can barely tie their own shoelaces working out. "Oh no," she says, shaking her head. "I think it's great. It's just something a bit different, isn't it? He already plays football and other sports, but still has so much excess energy that I'm happy for him to do anything to burn it off."

And isn't it just a cynical ploy to encourage tiddlers to get hooked on paid-for exercise, resulting in pricey gym memberships later in life? "Oh noooo," she says again. "I've heard a lot of people saying that, but they're obviously not the kind of people who get enjoyment from exercise. There are so many overweight kids and adults, that if we can try and set an example for them, if they can tag along, it's got to be a good thing."

Alison Lynch, Arron's mum, is equally delighted. "His hand-eye coordination has improved lots since he started coming here," she says. "He's still not very good at balancing, but I've noticed a definite difference."

But wouldn't children be better off playing outside? Dr John McKendrick, a child-play expert from Glasgow Caledonian University, has criticised parents who "wrap their children up in cotton wool" rather than letting them play outside with friends. Perhaps surprisingly, he considers the children's gyms to be a positive step in the right direction, but recognises the limitations of such initiatives: "In an ideal world you wouldn't need them and all children would be able to play outside in a safe environment but, as that sadly isn't the case, such facilities are a helpful addition to a wider programme of encouraging physical activity.

"But such initiatives do not present a panacea to the national obesity problem. It is important that exercise becomes part of daily life."

At £2.50 a pop, Magnum sessions are beyond the budget of many families. It's also questionable whether public money is best channelled into such exclusive ventures - the two gyms are partly funded by grants of £53,000 from the Big Lottery Fund and North Ayrshire Leisure. Plus, its slightly out-of-the-way location inevitably means that most children are ferried in by car - a great irony that McKendrick points out: "This perpetuates existing patterns of transport, and the nationwide problem of there being not enough independent mobility of our children."

It's easy to tut about kids being cooped up in unnatural environments as they work out like munchkin hamsters, but the fact is that many children simply don't like being outside. Brenda Walker has brought her two children, Kirstin, 11, and Alistair, nine, to the gym. She says that, while her daughter is sport-mad, it's more of an effort getting her son off his backside. "He's a PlayStation boy," she says, with a sigh. "I'll try anything to wrench him away from that."