The research was carried out by magazine Woman and the National Obesity Forum.
It said the main reasons for kids getting too big were:
Lack of exercise at school
Parents being too worried to let their kids play outside
More kids spending more time watching TV and playing computer games
The survey revealed that more than half of the UK's kids have a computer, TV or games console in their room.
Dr Ian Campbell of the National Obesity Forum said kids need to think more about their health.
"If we can encourage good nutrition and physical activity, the weight issue will often look after itself," he said.