Actually, in regards to the EM/Peds combined residency, it does not qualify you to sit for the Pediatric EM exam. You must complete a fellowship after your EM residency in Pediatric EM which is 2-3 years depending on the program. So, if you did a 5 year combined EM/Peds residency, to be certified in Pediatric Emergency Medicine specifically, you would have to do another 2-3 year fellowship.
Check out the article in the Feb/March 2003 EMResident (EMRA publication) written by Dr. Lance Brown who is currently finishing his Peds EM fellowship at Loma Linda University.
Also, SAEM's site has a link to EM Fellowships and specifically Pediatric EM listing all the programs who offer the fellowship.