I find many of my clients were consuming way too much of this as well as processed foods. Make sure you choose good quality protein at every meal. This includes chicken, turkey, fish, tuna, eggs or protein powders like soy or whey. These three factors will help you lose your body fat. If at all possible, I prefer whole foods as much as possible.
Protein is the key.
If your teen can eat scrambled eggs with turkey bacon, for example, every morning with a piece of toast, it would be great. You could cook the turkey bacon the night before. My favorite thing to do is break up the turkey bacon and add it to the scrambled eggs. You could even fill a flour tortilla with this mixture as well. The key is to get good quality protein every morning instead of relying on processed cereals or junk.