Exercise is key to staying healthy
Parents vastly overestimate the amount of time their children spend exercising, research suggests.
On average they claimed their offspring took more than two hours exercise a day - while the truth was less than 30 minutes for both boys and girls.
The government recommends at least an hour a day - but specialists say it is impossible for parents to work out how much their child is doing.
The study was published in the journal Archives of Disease in Childhood.
One in three 11-year-olds in the UK is said to be overweight or obese, using the body mass index method of measurement.
The 60-minute recommendation was introduced in an effort to stave off the advance of obesity driven by unhealthy eating and sedentary lifestyles.
However, the evidence to support it is less clear and the latest study, by scientists at the Universities of Glasgow and Newcastle, suggests that monitoring it is equally problematic.
They fitted 130 six and seven-year-olds with an "accelerometer", a portable recording device worn on a waist belt, and left it there for a week.
This measured exactly how much the child was moving during the day, working out how much time was spent on vigorous activity, such as brisk walking, running and sport.
They then asked their parents how much exercise they thought that their children had done during the week.
The parents provided an optimistic picture - with 83% of boys and 56% of girls reported to be meeting the 60-minute guideline.
Off the mark
Unfortunately, the accelerometer readings showed that in fact, only 3% of boys and 2% of girls had met the target.
Parents claimed an average of 146 minutes of moderate or vigorous activity a day, compared to an average 24 minutes offered by the accelerometer.
The parent questionnaires did not even manage to pick out accurately those with lower levels of activity from those who exercised more.
The study authors called for "marked improvements" in the way physical activity was measured to meet future "public health challenges".
They suggested that more research on the potential benefits of "light exercise" - which made up much of the playing activity of the children measured - might be worthwhile.
Professor Terry Wilkin, from the Peninsula Medical School in Devon, and a researcher into child exercise, said the results were "unsurprising".
He said: "What this shows is that parents really don't have a clue what their children are doing in terms of exercise - they spend most of their day at school, so how would they?
"If put under scrutiny, they will hopelessly overestimate activity levels.
"There is absolutely no basis for the current 60-minute recommendation and, frankly, we should leave that sort of information behind when we are trying to research the exercise levels of children."
Maura Gillespie, of the British Heart Foundation called the findings "deeply worrying".
She said: "In order to really encourage children to be more active, we believe it is crucial that the environment around them allows for daily exercise.
"The government needs to ensure our streets are attractive and safe for cyclists and pedestrians, encouraging more children to cycle and walk to school safely.
"Neighbourhoods, parks and green spaces need to be well designed and maintained to encourage children to play safely outside."
Professor Alan Maryon-Davis, president of the Faculty of Public Health, said: "This study shows just how inadequate our current information on children's physical activity is.
"Government boffins urgently need to come up with an accurate way of monitoring kids' exercise habits."