there are two ways to be pediatric emergency medicine certified.
one is to do a three year peds residency followed by a three years peds em fellowship. you can only see pediatrics in the er (i.e. you can not see adult patients).
the second is to complete an em residency, then do a 2 year peds em fellowship. you would be able to be board certified in both emergency medicine and pediatric emergency medicine. in other words, you could see both adults and peds in the er.
finally, if you complete an emergency medicine residency, then you are qualified to see both adults and pediatrics in the er. most likely you won't be working primarly in a children's hospital er (as those are staffed mainly by pediatric emergency medicine trained physicians). i'm not sure if that's clear but basically if you do a em residency (3 or 4 years) than you can see whatever walks through the door. if you practice in a community er, you will probably be seeing a good portion of peds as well. of course, i think that doing a fellowship in peds er makes you much better at peds. if you look at the curriculum at most er programs, there's only 3 mos dedicated to peds er... and then whatever you happen to see in the "adult" er's.