How are pediatric hematologists and oncologists similiar? Are they the same doctors?

Yes. Yes, they do at least 3 years of combined training in both areas.
Same. Unlike internal medicine physicians who have separate training pathways and board certifications for hematology and oncology, in pediatrics it is one training program and one board certification. Although pediatric hematologist-oncologists are trained in both, some specialize in one or the other.