These are two very different things, I think you need to find which direction is the best direction, but you still have time to do this. The amount of time and money to be a pediatric oncologist is quite extensive, and you wont have enough time to do the job of a social worker and physician. When you try to do two things at once, one or both things inevitably aren't done well. Go to college, knock out your general pre reqs and see how you do. Gain clinical exposure early and get a sense of your passion. Understand that medicine has many sides you dont see as well. Rank the things that you value most, desire for family, money, work-life balance, etc and see if that rank list applies to the direction you want to go. You have to vastly different career paths in mind, the first step is knowing which is the right path for you.
On that note, I didnt even know i wanted to be a physician until early 20s, never would have thought it. And here I am in my early 30s as a psychiatrist.