I have an overweight 9 year old. I've posted on here before about it. Lots and lots of people don't get it at all - basically say "why don't you feed her healthy stuff and get her to exercise". I would have said that 9 years ago, I am skinny, I would always have thought it was simple to stay slim until I had to parent my daughter!
My DD was 10lb 1oz when she was born, has always been chunky and just has a huge appetite. I am convinced it is something to do with her brain chemistry - she doesn't have an off switch like my other DD. So for about 3 years now I have not bought ANY crisps or biscuits, or anything processed and not much bread either. She has porridge or eggs for breakfast, a small packed lunch of protein/veg/plain yoghurt/piece of fruit type of thing, and either a "picky tea" (fruit, veg, dips, piece of ham etc) or a cooked tea which is fish/veg, baked potato/limited pasta and sauce etc. Mostly homemade. She also swims for a club 3 times a week, runs twice a week and does a fair bit of other activity. And she is still overweight - but only a bit. All I can think now is that if I wasn't doing what I was doing she would be seriously overweight, and her build and appetite is just something she and I need to learn to live with. I am hoping that by setting habits in place now, particularly the habit of doing sport, which she loves, that they will just become part of her life in the long term.
As for the talking about it, I tackled it when a particular sport that she does required her to be under a weight limit. I explained that she wouldn't be able to get heavier over the coming year and that together we would need to work on just sticking to the really good food, and just one portion. She has taken it on board and she and other DD do understand what good food is and what they shouldn't eat too much of. I never talked about how she looks, it is all about keeping fit and healthy for her sport and minimising the amount of weight she puts on in the coming year or so. Her size and shape are a big benefit to her in many ways, she is very strong, so I make sure she knows that her body is good because it is fit and strong. Personally I think a 20 minute DVD a day sounds deathly! If it's something you and she would enjoy together perhaps have a go, but taking up a sport with friends might be more fun?
It is so hard because I can see that food is becoming a bit of an issue - both DDs look forward to parties, trips and holidays mostly because of the ice creams, cakes and sweets they will get! So I am slacking off a little bit otherwise they are going to get completely hung up about it all.
That is all about me me me, but I can only talk from my experience with my children, I don't have any other expertise on children's nutrition. You will find there is barely any help out there - basically all it says is not to feed children junk food and to get them to walk places a bit!!