I was just reading the "overweight children" thread.
When my DD had a routine op, she wasn't supposed to run about or do any sports for 6 weeks. She got a bit plump so, without her realising, I changed the food she ate in subtle ways - low fat yogs instead of full fat, more fruit/less sweet stuff (although she didn't have THAT much to start with.
She's now a normal 7 year old girl - not a skinny (like DS!) but a 7 yo with a little tum. Just right
So there.