If this isn't an ad, then I'd do nothing!
I was an active kid - always out playing football, riding my bike, climbing trees etc... yet was fat. I didn't eat any more than other kids my age.
My father always referred to it as puppy fat.
As soon as puberty started to kick in the weight fell off me. Over the course of the summer holiday during my 1st year in secondary school and starting my 2nd year it was a though I was a different person.
My 8 year old daughter is 'chunky' and yet eats bugger all and has loads of exercise, so despite Mrs Flop's constant worrying about her weight, it doesn't particularly bother me as I suspect as soon as she hits puberty it will fall off her as well.