What is the difference between an oncologist and a pediatric oncology doctor?

Training and age. They are kind of 2 fields. Pediatric oncology is only for children and teens and has special challenges. Most medical oncologists see only adults.
Patient age. Medical oncologists are internists that treat cancer in adults. Pediatric oncologists are pediatricians that specialize in the cancers of childhood. The diseases and their treatments are actually quite different in adults versus children.
Adult versus Child. Adult oncologists take internal medicine residency training after medical school. They then do a fellowship in adult oncology. Pediatric hematologist/oncologists take pediatric residency training followed by pediatric hematology/oncology fellowship training.