Children under the age of five should exercise for at least three hours a day, new UK government guidelines suggest.
Babies should be taken swimming and play on "baby gym" mats while toddlers should walk for at least 15 minutes of routine journeys, such as to nursery, chief UK medical officers will say.
The exercise guidance, to be issued this week, targets under-fives for the first time and comes amid growing fears over childhood obesity.
"For children that are not yet walking, there is considerable evidence that letting children crawl, play or roll around on the floor is essential during early years," England's chief medical officer Sally Davies said.
"Play that allows under-fives to move about is critical and three hours a day is essential," she told The Sunday Times.
"I think there are parents who are not aware how important it is for their children to be physically active for a minimum of three hours.
"Other parents are very busy and may not see how important it is to get that prioritisation and balance right."
According to NHS figures, nearly a quarter of youngsters aged four and five are currently overweight or obese.
Experts predict that by 2050, this could apply to 63 per cent of children.
Professor Fiona Bull, chair of the scientific committee behind the guidelines and co-director of the British Heart Foundation National Centre for Physical Activity at Loughborough University, said parents should "turn the TV off".
The advice follows warnings earlier this month that England faces a liver disease "timebomb" because so many children are overweight.
Professor Martin Lombard, national clinical director for liver at the Department of Health, said a culture of overeating was putting the lives of more than 500,000 young people at risk.
Interesting.... though I think they should refer to Movement Through Play - I don't think they mean formal excercise classes