Australia's treadmill tots

Children as young as three have personal trainers and mini-exercise machines to help fight childhood obesity but health experts say they need to go out and play, not hit the treadmill.

At Sydney's Oxygen fitness studio, child-sized steppers, rowers and spin bikes are used under the guidance of a qualified personal trainer.

Studio owner, former Cronulla Sharks player Ben Sammut, said children worked out for an hour on brightly coloured state-of-the-art equipment while their parents exercised with a trainer in the next room.

He said his Oxy-Kid classes, which began last month at Miranda, in the city's south, were full.

"Kids these days are not as active as they once were, so our classes are about having fun and being active," he said. "We don't talk explicitly about being obese but we talk about healthy food options, such as eating five [pieces of] fruit and vegetables a day."

About 15.2 per cent of Australian preschoolers are overweight and 5.5 per cent are obese, the Growing Up In Australia study found last year.

Evidence shows these children will probably be obese adults and at a higher risk of developing diabetes and cardiovascular disease.

Louise Baur, from Sydney's Children's Hospital, said structured exercise for youngsters risked creating "battery-farmed children", who were not encouraged to use their imagination. "It is an inappropriate way of encouraging physical activity in kids," she said.

Professor Baur said fears about "stranger danger" led to more children being kept indoors when they should be out playing. "I feel angry that we live in a world where we make it harder for kids to do what they've done for millennia - play."

Child-care centres and preschools are also doing more to tackle obesity.

Headstart Early Learning has five Sydney centres, which general manager Kat Wieczorek said were designed to get even nursery-aged infants physically active.

Ms Wieczorek, a former lecturer in childhood studies at the University of Western Sydney, installed bikes, cycle tracks, beams, climbing frames and obstacle courses to develop children's motor skills and lift activity levels.

Jack Rule, 4, of Lilli Pilli, takes part in Oxy-Kids twice a week.

Mum Robyn said the classes had imbued a sense of the importance of healthy eating and exercise.

"Before going to the gym he'll ask for a piece of fruit because his trainer has told him it's healthy, so it makes my job easier," she said.