The Department of Health has launched its biggest ever survey of children's health.
The study, How are the kids?, will see around 11 million families giving information about their eating and exercise habits.
In return, parents will get tailored plans to help them eat more healthily and become more active, under the department's Change4life initiative.
If current trends continue, nine out of 10 children could be overweight or obese by 2050. At present, almost a third of 10 and 11 year olds are overweight or obese.