Seven out of 10 parents think their child takes plenty of exercise, despite only one in 10 doing enough, a poll has found.
Research for the British Heart Foundation (BHF) found there was a "reality gap" between what parents believe and what actually happens in day-to-day life.
The charity released a report called Couch Kids on how a lack of exercise is fuelling childhood obesity.
Children are taking no more exercise than a decade ago and obesity levels have risen dramatically over the same period.
In 1995, 11% of boys and 12% of girls were overweight or obese, rising to 17% of boys and 16% respectively in 2007.
NHS data shows about one in three young people are currently overweight or obese.
The BHF survey of more than 900 parents found 71% believe their children are "active enough" but only 11% of youngsters are active for 60 minutes a day, as recommended by the Government.
Dr Mike Knapton, associate medical director at the BHF said: "Mums and dads need to take off the blinkers 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.
"Parents have such an important role in helping and encouraging their kids to be more active by building exercise into their everyday lives - whether that's taking part in team sports, playing outside with their friends or walking to school."
