In the first study to rigorously track the movements of preschoolers, scientists found that the average 3-year-old is physically active for just 20 minutes a day, well short of the recommended hour a day for that age.
In The Lancet study published this month, scientists from the University of Glasgow in Scotland recruited 78 children. Each 3-year-old wore an "accelerometer," a matchbox-sized monitor clipped to the waistband, for a week.
The device, worn from the time the children woke up until they went to bed, gave minute-by-minute readings of the children's pattern of physical activity and the number of calories burned.
The toddlers were burning about 1,300 calories a day - less than the 1,500 calories recommended.
While the problem is one of an imbalance of calories eaten and burned up, experts believe the main reason is that children are not getting enough exercise.
"There are really only two possibilities, reduced activity or increased intake. None of the dietary assessment data indicate that children are eating more. Adolescents may be eating more but young children are eating less," said the study's leader, John Reilly, a physiologist at the University of Glasgow.
"A 3-year-old 25 years ago was eating 25 percent more than a 3-year-old today," he said. "But physical activity levels have dropped quite dramatically over the last 15 or 20 years."
In the study, the children were spending between nine and 10 hours of their waking day hardly moving at all.
"They may well have been doing a bit of fidgeting, they may have been speaking to their parents or among themselves, but they were just not moving enough to put up the number of calories burned beyond what it would be if they were just resting or sleeping,"
The children spent 20 minutes a day in moderate to vigorous activity - the type of activity that would get them feeling slightly warm and slightly out of breath, such as running around, walking to keep up with an adult and most types of outdoor play.
All-day television and recorded videos are a major culprit, Reilly said. Outside the home, children are also much less active than they used to be.
"Many more journeys are made by car and among the 3-year-olds, a fair number of them are being taken around in strollers when they could arguably have been walking," Reilly said.
Another element is recent concerns over safety. Some local authorities in Britain have banned children from bringing balls into playgrounds while others prohibit tree-climbing.
"There needs to be a balance. Perhaps we've taken the health and safety agenda a bit far," Reilly said.
The dangers of a sedentary childhood go beyond obesity, experts said. More active children tend to be better behaved and scientists suspect that more active children learn more effectively, perhaps because physical activity is a stimulus to brain development.
"The increasingly sedentary nature of U.K. children is not unique and is being seen in most countries around the world," said James Hill of the Center for Human Nutrition at the University of Colorado.
Small changes in behavior are all that is needed, said Hill, who was not connected with the study.
In the days before videos and TV, young children didn't sit for hours staring at the wall, Reilly said. "They were perfectly capable of finding other ways to amuse themselves. They had imaginations. They still have imaginations," he said.
By Emma Ross