What would be a good present for a child cancer patient?

This may depend on. The age of the child and his or her interests. For those children who spend a lot of time in the hospital for their treatments, games and activities may be appreciated. For others, it's a stuffed animal or something to give them support while they're getting therapy. The siblings often get ignored so remembering them can be helpful too.
Anything they want. Children diagnosed with life threatening illness need and deserve any distraction from the day-to-day treatment and sickness they must endure. Splurge, but make sure it's age appropriate.