A close friend of mine has a similarish issue with who DD who is 11yo. She always had a "large appetite" but seems to have gotten into this routine where straight after breakfast she is hankering after a snack and asking what time lunch is. She is not fat but definately quite solid in build and heading that way.
My friend has started to take the family swimming twice a week. Got DD to join a street dance class once a week as well. She makes a concious effort to have a family outing at the weekend to a park etc where they are out and about and walking - even if it is just a bit of a stroll.
She no longer has ANY crisps, biscuits, cakes in the house except on special occassions and makes the odd slimmingworld cake. My friend is on the SW plan herself. Some days she serves up her SW meals to her family other days she does not but the principles of this plan is going back to basics and avoiding processed, packet, sauces and meals and using low fat, low sugar ingredients. Her fridge is stocked with low fat/fat free yogurts for snacks and the fruit bowl kept full. The rule in her house is that the kids can help themselves to the fruit bowl but anything else they have to ask for.
She has not told her DD that she is on a diet in fear of knowcking her confidence and has made these changes to the whole family and is hoping her younger DC grow up not knowing any different. There have been a few moans apparently but friend says she is on SW diet and that she cannot afford or has the time to cook too many seperate meals - a bit of a this is how it is so tough.
Oh another change she has made which she reckons has helped lots is replacing large loaves of white bread for smaller loaves of brown bread. Her DD had a tendency to just go and make toast when she was hungry.
She is doing all this without mentioning her DDs weight and so far the results seem to be working. There has been a small and slow weightloss - well, a few items of clothing have become looser and better fitting - friend has not actually weighed DD.
This is my best friend and she wishes she had made changes like this a few years ago now. Hope this helps and is not too condescending. I dont mean it to be - but its all kind of common sense stuff, but what I wanted to try and get across is that it can be done subtley without the need to tell your DD she is on a diet if you don't want to.