You probably should have them evaluated by their pediatrian to determine if this disorder can be treated: http://www.mayoclinic.org/diseases-conditions/gynecomastia/basics/definition/con-20028710.
Agree with mark, but are. They overweight?
Some food treatments can cause this problem. I would speak to a nutritionist,
Some psychotropic meds also cause this problem, http://www.drugwatch.com/risperdal/gynecomastia/. I would start with the MD and have him/her recommend a nutritionist if needed.