Most pedi EM (henceforth known as PEM - as most folks in the field will say) work harder than adult EM. Increasingly, PEM fellows are from the adult world - but you're right, most candidates are from pedi residency programs.
And Dr. McN is also right - PEM docs either make less money or work more. Why? There are less of us and a huge demand. You would think the 'huge demand' would lead to a large paycheck, but it generally just leads to more shifts. Remember, most of the PEM attendings right now came from straight pediatrics - and peds people are used to making less money. So the step up from straight peds to PEM is enough that most folks are happy.
Given the increasing number of EM folks getting certified in PEM that may change. It is a growth market, so there is nowhere to go but up. I'd say get in NOW - PEM docs will be making money hand over fist sometime in the near to mid-term future. Hospitals are finally realizing that it makes economic sense to have treat emergent kiddos effectively the first time.
If you haven't done residency yet, tho, the caveat is that PEM is getting very very competitive. Pedi people want to do PEM, EM folks want to do PEM, so whatever you do - do it well.
Good luck.