Precisely the reason Weight Watchers doesn't work very well. They let you eat carbs.
The best weight loss approach for kids is a low carb, normal calorie diet combined with exercise, for the entire family. That means getting rid of bread, pasta, potatoes, rice and junk food. Eat lots of good protein with healthy fats and some fruit.
While diets in general have a low success rate overall, there are many, many people who have successfully lost weight and kept it off. Good luck!