I'm not a peds EM fellow, but work in an ED that has them. There are a couple of approaches you could take, depending on what you want to do long term.
EM/peds combined programs offer good training in both EM and pediatrics. The catch is that you can't sit for the specific peds EM boards afterward, though some would say you don't need to after such training.
Most people I know doing peds EM did 3 years of peds residency followed by 3 years of peds EM fellowship. You would then be boarded in pediatrics as well as peds EM. During that fellowship you get some training in the adult ED, but long term, you would only be working with kids. In other words, you couldn't then go work in an adult ED. To me (and most peds residents) that's just fine, because I can't stand adults. 😛
Peds EM isn't using ultrasound as much as adult EM, but that's likely to change in the next few years. It's also one of the more competitive peds fellowships.
The other path is to do a 3 (or 4) year EM residency and then do a 2 year (I think) pediatric fellowship. The ones I've seen take this route are also well trained, being boarded in EM and peds EM, but not general pediatrics. This route offers a lot of flexibility in terms of where you want to work, and will let you continue to care for adults if you want. It may affect your ability to be hired at a dedicated peds ED that sees only kids, as those places are primarily hiring peds-->EM guys, but I can't say that for certain. You would also have good ultrasound experience and training.
