Only one in 10 children gets the recommended daily activity
Parents have big misconceptions about the amount of exercise their children do, the British Heart Foundation says.
It says 71% of nearly 1,000 parents polled think their children are "active enough" but only 10% of their children say they do the recommended amount.
The charity says parents need to take the "blinkers off" about how active their children are to prevent obesity.
The National Obesity Forum urged local councils to improve the quality and range of affordable sports facilities.
The BHF questioned nearly 1,000 UK parents with children aged eight to 15.
It produced a report called Couch Kids which shows that while the number of obese children has risen since the mid-1990s, there have been no major changes in children's physical activity levels over the past decade.
The BHF is calling for urgent action to ensure children can get fit and active easily with:
- Guidelines on recommended levels of physical activity for all age groups
- Schools maximising the opportunities for physical activity
- Collection of data about physical activity levels across the UK by local authorities and national governments
Dr Mike Knapton, associate medical director at the BHF, said: "Mums and dads need to take the blinkers off about how active kids need to be in order to keep their hearts healthy.
"Kids need to get moving more, yet we've been standing still for the last decade.
"Children aren't really any more active than they were 10 years ago."
Dr Ian Campbell, of the National Obesity Forum, said: "Parents, carers and children should try and find ways of being active together, at home, at school and at play.
"The barriers to increased activity are many, and complex.
"In addition to parents being more involved we need to ensure local councils are aware of the need to provide greater opportunity for all children to be more active, with better, more affordable sports facilities and safer, cleaner, well-lit streets."
A Department of Heath spokesperson said: "Tackling childhood obesity is a priority for the government.
"Emerging evidence suggests childhood obesity rates may be levelling off but they are still too high.
"If we're going to turn the tide on obesity for good, our children need to be active for 60 minutes a day - including what they already do at school.
"We are taking tough action and investing £372m to address obesity and to prevent people becoming overweight in the first place.
"Many of the recommendations in Couch Kids have already been addressed in the government's obesity strategy and the PE and School Sport for Young People strategy."