As others have mentioned, it's probably better to focus on healthy habits and a healthy body image than on dieting at this age. We've had to deal with body image in our house. Here's our issue:
We have two boys, 6 and almost 13. The 12 year old is healthy, but is a bigger build than our younger son. He is very health conscious and always has been. He excercises and loves to read food labels. My husband and I are not obsessed about these things, but my son is really into it, and he's healthy. Well, our youngest picked up on it last year. He is very tiny - short, small bones, very, very thin. He started worrying about how much sugar he was eating. We don't even keep sweets in the house, but he worried about sugar on all of the labels. He gets plenty excercise, and as I said, he's tiny, but he started thinking he was fat. He would get very upset about it, and wouldn't eat certain foods because he thought it wasn't good for him (like Frosted Cheerios). I couldn't believe this was coming from a 5 year old boy. We did everything we could to promote a healthy body image, and had to be careful about everything we said in front of him. I'm small too, but I'm still a woman ;-) , (you know we're always hard on ourselves) and I had to be sure I never said anything negative about myself in front of him. Eventually I had to take him to the doctor and have the doctor and the nurse talk to him about how he needed to eat healthy foods, but that some sugar was alright sometimes too. They talked to him about how he needed to grow so that he would be healthy and strong. Eventually he got out of that phase, but we are still very careful to focus on healthy habits rather than on appearance. It was quite an ordeal, and it went on for about 4 months.
I'm sorry this got long, but I thought this story might help you with your situation. We don't want our children to be too thin or too heavy, but it's important to approach it from the angle of being healthy, not appearance, which I know is what you are doing, but they might not understand that if you aren't very careful about it.