I'm not positive about this, but I think that for pediatric oncology one does a three year pediatrics residency, followed by a two year fellowship for pediatric oncology or a three year fellowship for hematology-oncology, which seems to be more common.
Pediatric oncologists have "three patients", the child, and the parents, who seem half out of their minds when their child is diagnosed, but often rally to become very well acquainted with the child's diagnosis, treatment, and care. Parents respond well to being treated as partners in the decision-making process, and often are motivated to learn to provide as much of their child's care themselves as possible.
I think that pediatric oncology is a good job for a "people person" who also knows how to change gears and re-focus with hobbies or other interests when off duty so that the job doesn't cause burn-out. It can be heartbreaking but also rewarding, and provides opportunities for longer-term, more in-depth relationships with patients than some specialties.