Can a pediatric oncologist have to be a surgeon?

No. Classically, pediatric oncologists are pediatricians who have completed special training in the care of children with cancer. This special training is called a fellowship and typically lasts three or four years after completion of a three year pediatric residency. The surgeons who care for children with cancer are not referred to as pediatric oncologists.
No. Training after medical school to be a surgeon is different than training to be a pediatric oncologist. Although not a surgeon, pediatric oncologists typically perform to minor "surgical" procedures; lumbar punctures (spinal taps) and bone marrow aspirations and biopsies.
Two different doctor. Pediatric oncologists are pediatricians who have specialty training in treatment of cancer. They work with surgeons who can assist them in removal of tumors and other procedures to further the treatment of the cancer.