Isabella...a lot depends on what track you want to take. My suggestion is to first concentrate on your High School and future college plans. While TODAY you are interested in a career as a Pediatric Oncologist you will find that as you progress through college, medical school, residencies, and specialty training the competition is fierce. I have known many physicians whose initial intentions were dramatically different from their final field of practice.
So in the end...is High School Physics a requirement to be a Pediatric Oncologist...no. But is it something that will benefit you and make you more competitive for your pre-med program...absolutely.
HIGHLY recommend you do some volunteer work in an area where you are exposed to physicians who practice Pediatric Oncology...this way you will know if this is REALLY a career you want to pursue.