The accredited peds fellowships, which all require completion of a 3 year pediatric residency first, are:
Subspecialty Length
Adolescent Medicine 3 years
Allergy & Immunology 2 years
Cardiology 3 years
Child & Adolescent Psychiatry 2 years
Critical Care Medicine 3 years
Endocrinology 3 years
Gastroenterology 3 years
Hematology & Oncology 3 years
Infectious Diseases 3 years
Medical Genetics 2-4 years
Neonatal Perinatal Medicine 3 years
Nephrology 3 years
Pediatric Emergency Medicine 3 years
Pulmonary Disease 3 years
Rheumatology 3 years
Some may have an alternative path as well (e.g. adult EM can do a peds EM fellowship, FM or IM can do an adolescent med fellowship). In addition, there are a number of non-accredited fellowships pediatricians can do, for things like peds hospitalist or various research focuses. There does also exist a mechanism to do a research track and start one of the above fellowships without completion of the full 3 year peds residency, but those tracks generally require a dedicated extra 2-3 years of research in exchange for cutting off 1 year of residency.
That said, if you're truly "pre-health", this is a question that is super early for you to be worried about. This forum is for people who are already almost done with medical school and applying for residency, in residency, or beyond that.
Pre-allo (for premeds) is over at
http://forums.studentdoctor.net/forums/pre-medical-allopathic-md.10/