How do you help a child lose weight?

Burn calories: Swim, bicycle, abdominal crunches, find 300 calories a day you don;t need to eat e.g. Bread. If you eat meat, make it lean, take off the skin of chicken, no fried food, eat more fish and seafood, eat green and white veggies cooked with garlic and oil. Rice and beans(lentils and rice) are good sources of complex carbs.

Correct nutrition: A well balanced diet, plenty of protein and complex carbohydrates and small amounts of unsaturated fat is a good start. Fat has twice the amount of calories than carbohydrates or protein and sweets are simple carbohydrates that produce a type of energy that doesn't last long. Also size of the portion is very important, keep it appropriate. No juice or soda, no refills. Lots of aerobic exercise.

Feed less,exercise!: More physical activity is necessary, and you must make sure you do not serve high caloric meals with lmiited additional servings. That means don't make too much. Sanitize your house from having useless calorie, junk foods. And you need to set a good example.