You can probably start with just a high school diploma if all you do is watch them.
You would need some college for anything with more responsibility. You could be a therapist, nurse, doctor, and anything in between.
You should visit a children's hospital in your area to find out. I don't know as much about orphanages, there aren't that many here in the US anymore.
If what you like is working with needy children, there are hundreds of things you could do. Most of them need some training after high school.