I can tell you my own experiences. When I was 4 I started complaining about chest pains. Everyone blew it off. I would have them off and on. Had them my entire childhood. When I was in college, I complained to a doctor and he sent me to a cardiologist. They assumed it was nothing (which it wasn't anything major), but he did diagnose me with mitral valve prolapse. I went on a medication and the pains went away immediately.
Here's my mom advice. If your child is complaining about chest pains, I don't think it hurts to see a pediatric cardiologist and just have him checked out. At least you would have peace of mind.
Mom to 4 crazy kids and a paranoid rabbit