Don't nag your child about losing weight to make his weight class or fit into her prom dress. God and Mother Nature intended our bodies to be a certain healthy weight. Trying to override that weight can lead to horrible consequences. Dieting is the strongest eating disorders trigger we know of. Don't forbid certain foods.
Healthy eating has room for just about all foods in moderation.
In addition, learn what normal development and weight gain look like. Encourage normal, healthy development in your children. Encourage healthy eating, not dieting. There is a difference. Make mealtime pleasant.