Deciding if a child needs to go on a diet, say experts, depends on many factors:

*Start with height and weight tables. Children 20 percent over the norm for their age, height and sex are considered obese.

*Look at your own weight. Children with heavy parents are predisposed for obesity.

*Consider whether high blood pressure or cardiovascular disease run in the family. Extra pounds mean extra risk for children from these families.

*Check with your child's doctor before starting any weight loss program.

To help children lose weight, University of Pittsburgh psychologist Leonard Epstein offers this advice, drawn from five years of research on overweight youngsters:

*Record what your child eats for a week. Figure the number of calories per day. Then cut back 500 calories a day, never going below 1,000 total calories per day. This will allow the child to lose about a pound a week.

*Make sure children eat a balanced diet. Recommended: two 3 oz. servings of protein (for instance, 150 to 220 calories per serving of lean meat, fish or poultry without the skin); two 8 oz. servings of milk (skim milk, low-fat cheese or yogurt); four servings of grains (80 calories per serving of cereal, bread, rice, spaghetti); four servings (about 1/2 cup) of fruit and vegetables.

*Show youngsters how to use a calorie counter. Have them help you shop for groceries and when they request high calorie foods, ask them to look up the calorie count so they know why they can't have it. Likewise, show them what foods are allowed.

*Decrease sedentary activities. Among the least active: watching television, playing video games and working on computers. Put time restrictions on these activities.

*Increase aerobic exercise. Suggested activities: walking, swimming, jogging. Promote active lifestyle changes, such as using the stairs instead of the elevator.

*Keep daily records of what children eat and how much they exercise. At the University of Pittsburgh program, parents review children's records daily and help them make changes where necessary.

*Reward children for losing weight, but don't offer food, money or material possessions. Instead, give privileges. Or, spend more time with them. For instance, allow a child who lost five pounds to stay up an hour later on a weekend, or take the youngster to a movie of his or her choice.