We now have evidence that putting children on diets, although well intentioned, many times causes harm. Children learn to sneak and typically gain more weight. There is also evidence that daughters who have mothers dieting"with" them, and becoming too involved in their eating, have a greater risk of struggling with bulimia.
There is another way to handle concerns about your child's weight. Focus on healthy eating and more activity for the whole family and remove the focus from weight. Seek help from a professional for a clear understanding of approaching this subject with the greatest insight and skill and prevent problems down the road.
Below is a short interview with Dr. Ed Abramson discussing this issue in greater detail:
http://www.kqed.org/a/perspectives/R201104010735