Is it okay for a child donate a kidney to their parent?

It May Be: I think it depends on how well the tissue match test turns out for the donor kidney (the child) relative to the recipient (the parent). If there is not a good enough match, there can be problems with rejection. But otherwise, nothing else precludes someone from donating to a parent. In fact, i think it is common for close relatives with the correct tissue match to donate to other relatives.