What are the risks of children who lose a kidney from cancer at young ages and become teens?

Really good these days. It depends on the cancer, how soon it was discovered and the treatment. My college roommate went through this as a kid and has never had a recurrence. That was a long time ago and things are even better these days. Good luck to whoever you are writing about.

you can function just fine with one kidney

You can function near normally with one kidney. My son lost one a year ago to kidney cancer. He has no limitations on what he can do. I have heard that you should monitor your blood pressure because it can be high, but that is it.