well the most common specialty after plastics for pedes is craniomaxillofacial surgery--dealing c a lot of congenital anomolies. From what I have heard, it is very gratifying/rewarding (just like almost any other surgical peds specialty). Umm, as far as training, I believe it is a 1 year sub fellowship after plastics, correct me if i am wrong. So, how much ever it takes you to do plastics (5, 6 or 7, or more), plus another year for CMF.
Just like any other big peds subsurgical specialty, you are almost invariably bound to work at a big academic institute. Most parents want to take their kids to the best of the best, and usually the best of the best and the most expertise lies in big academic institutes. Subsequently, this will limit you to live in a fairly decent size city with a decent size children's hospital, thus limiting your geographical desires, if you have any. Thus, keep all this in perpective when you decide to really (sub)specialize in pediatrics.