I've heard there is a link between kids who are very tall/fast-growing for their age and increased cancer risk. is this true?

Unclear. : There are a many studies that have attempted to link height in childhood and the risk of adult cancer. A few have shown a link, particularly in breast & prostate cancer, though other studies have not. These studies have methodologic problems: usually case-control studies based on surveys & self-reported data. I would not worry about height, but following a balanced, nutritious diet with exercise.

Mostly no: Tall boys are supposedly at a slightly greater risk for the common bone sarcoma of teens. I've never seen anything else to make me think that there's a real risk in fast-growers for any other form of cancer, and that this urban legend makes people see faces in the clouds.