Less than 3 per cent of UK 11 year olds are taking enough exercise, suggests research published ahead of print in the Archives of Disease in Childhood.
It is recommended that kids spend at least an hour a day doing some form of moderate to vigorous physical activity, in a bid to promote good health and stave off the risks of subsequent obesity and diabetes.
The researchers monitored the physical activity levels of more than 5,500 11 year olds in the South West of England over seven consecutive days between January 2003 and January 2005.
The children were part of the Avon Longitudinal Study of Parents and Children (ALSPAC), which has tracked the health of more than 14,000 children since birth.
Each child was kitted out with a special piece of equipment (accelerometer), worn on an elasticated belt, which recorded minute by minute the intensity and frequency of physical activity.
The researchers were particularly interested in total levels of physical activity and the amount of moderate to vigorous exercise the kids were taking daily.
When the data were analysed, they showed that the children were around twice as physically active as adults, but they were still not active enough.
Boys were more physically active than girls, and they were also more likely to engage in moderate to vigorous forms of activity.
One in five (22%) girls averaged at least one bout of moderate to vigorous activity a day, lasting at least five minutes. This compares with 40% of the boys.
But both sexes spent most of their day in light intensity activities. Less than 1% of the children averaged at least one 20 minute bout a day.
And only just over 5% of the boys and 0.4% of the girls actually achieved current recommended daily levels of physical activity, equating to 2.5% across both sexes
Only sustained activity is likely to promote cardiorespiratory fitness, say the authors, adding: "It is a sobering thought that children's activity levels actually peak at around this age  and decline precipitously during adolescence."