How do I set up Undertale on an iPad?
Sorry to disappoint you but I don't think something like that is possible on an iPad.
From what I know you can't easily port something from a PC to something like an iPad unofficially. You could either hope for the developer to develop and release a port to iPad someday or since your nephew isn't that computer savvy there's still an official Nintendo Switch port of Undertale that exists.
Hopefully this answer can help you.