Article from About.com...

Although trying to help overweight children lose weight is important, even more important may be trying to prevent them from becoming overweight in the first place. This too is not easy, but something that needs to be started in early childhood, especially if your child is at risk for becoming obese, such as having overweight parents.

Targeting the behaviors that lead children to become overweight can be helpful in preventing your child from becoming overweight. These include unhealthy eating habits and a lack of physical activity and exercise.

Tips, both to prevent obesity and help your child lose weight, include:
• Limiting the number of calories that your child drinks. For example, many kids drink too much juice and soda each day. Sticking to the usual recommend limits of 4-6 ounces of 100% fruit juice for children under age 6 years and only 8-12 ounces for older children can help to limit excessive weight gain.
• Limiting the amount of milk that younger children drink. Although drinking milk is important and it is a good source of calcium, too much milk can lead to your child becoming overweight. Obesity often starts in early childhood, with a common scenario being a child who drinks too much milk. Children usually only need about 16-24 ounces of milk each day.
• Avoiding frequent meals of fast food.
• Don't 'super size' your child's meals. A common problem that contributes to overweight children are meals with portions that are too large.
• Don't force younger children to 'clean their plates.' An important way to help children learn to eat healthy is for them to know that they can stop eating when they are full.
• Encourage regular exercise and physical activity in your children each day. This may include going for walks as a family, playing outside, riding a bike, or participating in organized sports, like soccer and baseball.
• Limit inactivity by setting strict limits on watching television and playing computer and video games.
• Avoid allowing your children to eat while watching TV. Instead, limit meals to the dinner table.
• Don't put too much of a focus on what your child eats. Remember not too restrict calories and instead, offer a healthy diet with 3 healthy meals (don't skip meals, especially breakfast) and a few snacks, and allow occasional treats. Talking to your child too much about calories, fat and dieting can actually cause more harm than good, leading to eating disorders.
• Know what your child is eating and where his calories are coming from.

And also important, be a good role model for your children by eating a healthy diet and exercising regularly. Keep in mind that a healthy diet is usually low in saturated fat (<10% of calories) and cholesterol (<300 mg/d) and moderate in total fat (<30%–35% calories).

--http://pediatrics.about.com/cs/obesity/l/aa012503a.htm

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