possible success, I would highly recommend this book
http://www.amazon.com/Child-Obesity-...3379317&sr=8-2
It is written by a doctor who specializes in childhood obesity. He explains concrete steps that parents can take to reduce or prevent childhood obesity.
His principles are not a diet, in fact, as one of the posters mentions above, there is a lot of evidence that managing kids' weight problems by trying to control what they eat is actually harmful-- it's called "restrictive feeding practices," and it's considered one of the underlying causes of obesity.
As well as I remember, he has five basic principles:
1. no sugary drinks (including fruit juice) apparently that's a HUGE source of younger kids' excess calories.
2. At least one hour of vigorous play time every day--not organized sports, but just playing.
3. Limit TV and video games to no more than one hour per day.
4. No more than one fast food meal per week.
5. Serve meals family style. Avoid restrictive feeding practices.
Number five means that if you are going to serve dessert, you let the child decide how much to eat. You put healthy choices in front of the child, but then don't comment on how much they eat, and don't tell the fat child not to eat certain things that everyone else is eating. If you eat cake, the child gets to eat it too. If you don't want the child to have cake, don't have it in the house.
I don't think I'm explaining the whole thing perfectly-- but the book is EXCELLENT and very easy to read.