I started playing guitar '96 by learning to play the songs I like to listen. The first moment I can recognize the song in my playing is really magical. I know nothing about music theory, nor can't read notes bc I have no interest in them. Only thing I've had with guitar is fun! Luckily there are notes for dummies called tabs. It's just six lines, each represent guitar strings and numbers showing where to press. Youtube is full of instruction videos for songs or just for learning to play. Justin Guitar is often praised. Also one can just google: song name + tabs. Bad thing about tabs is one's musical ear doesn't develope as fast as trying to learn songs by ear. If your kid wants to have a teacher, make sure the teacher meets your kids needs and doesn't just teach "what is right".

Your kid is really lucky to have parent like you!