Three-quarters of children are not getting enough exercise, while their parents vastly underestimate how much they need, a poll suggests.
The survey of more than 2,000 parents found 77% of children fail to do more than four hours of exercise outside of school per week, rising to 88% of children in low income families.
Public Health England guidelines say children aged five to 18 should engage in moderate to vigorous physical activity for at least 60 minutes a day.
Some 85% of parents did not know what the recommendations were, while 75% believed children need just 30 minutes a day.
The poll, from the British Heart Foundation (BHF), Diabetes UK and Tesco, also found 14% of parents saying their children did no exercise at all.
Some 85% of parents who earn £15,000 to £20,000 a year said some activities were too expensive, while almost a third blamed the lack of free or cheap facilities nearby.
And 37% of parents said it was a school's responsibility to ensure children get enough exercise.
Catherine Kelly, director of prevention, survival and support at the BHF, said: "Being inactive can lead to a multitude of health problems for children, including an increased risk of developing Type 2 diabetes and cardiovascular disease in later life.
"The benefits of physical activity outside of the classroom and within the community are vast, and as well as benefiting their heart health, it's fun too."
A national partnership runs between Diabetes UK, the BHF and Tesco to encourage healthier lives, including taking more exercise.