New government guidelines are to say that children under the age of five should exercise for at least three hours a day.
Medical chiefs will recommend that toddlers do 15 minutes of walking each day amid growing concern over childhood obesity.
Parents will be advised to take babies swimming and encourage them to exercise on play mats at home.
The guidance - the first to target the age group - will be unveiled this week.
"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."
Nearly 25% of youngsters aged four and five are currently overweight or obese, according to NHS figures.
Health experts have said this could reach 63% by 2050.