There are actually three ways to do it. A combined program, both residencies separately, or PM&R+peds fellowship. The advantage to the first two is that you would be boarded in both peds and PM&R. Some would say that it is important to be boarded in peds if you're going to be working in a children's hospital, all your colleagues will be. You probably will understand normal growth and development better etc. The snag with the combined program is even though it is 5 years you have to do a 6th year of dedicated peds rehab before you can sit for the peds rehab subspeciality boards/certification (from what I understand). I don't know if you do that year as an attending or a fellow.