Okay, so my mom says she saw this movie when she was a kid. Sometime between 1972 and 1976. It's about two little boys (one is black and one is white) who are friends. The little white boy gets sick so they go to a major city (she thinks it's either New York or London). She can't remember if the boy dies or not.

One scene she remembers is them rolling down a hill in a tire.

Can anyone name that movie?