If you want to work in a dedicated PediED, then my advice would be to go through a plain ol' EM residency and then do a peds fellowship...this will get you boarded in EM and boarded in PediEM. If you do the combined EM/Peds residency, you walk away boarded in Peds and boarded in EM, but NOT PediEM. You could probably find some places where you could work in a dedicated PediED, but it'd be easier if you had the fellowship. If your goal is not to be a pediatrician but rather to work in a PediED, then go for the fellowship after an EM residency. It's certainly a much shorter training period.
I had a similar thought last year and talked to my mentor about it, who is a graduate of a EM/Peds double residency (but not the PediEM fellowship). She told me that I should do the double residency if I could see myself doing both careers...as in, I could work happily as an EP or I could work happily as a pediatrician. As much as I loved kids in the ED, I just don't see myself as a pediatrician, but I could happily be an EP without the extra training in peds. So I chose to apply to straight EM programs.