For pediatric ER, do you do a pediatric residency and then a pediatric ER fellowship or do you do an ER program and then a pediatric ER fellowship? How many years does it take?

All pediatric EM fellowships are dually accredited by AAP and the ABEM. The AAP has ordered that any and all and, did I say, all peds fellowships - irrespective of what field - are 3 years. No exception.

EM peds fellowships are 2 to 3 years. Some programs are 3 years, no matter where you come from.

http://www.saem.org/services/fellowsh.htm#ped

This is from the horses' mouths (as it were).

Do you do a pediatric residency before the fellowship?

you can do either a peds or em residency.
from what i have seen, those that do peds first, only work in the peds er, and those that do em first all do shifts in the adult ed.