Ultimate Guitar is the gold standard for a reason. If you have a kid picking up a guitar, bass, or ukulele, this is their textbook. It successfully bridges the gap between 'watching a video' and 'actually playing,' which is a massive win for intentional tech use.

That said, it isn't a walled garden. The social features—specifically the forums and the video feed—are the 'wild west' of the guitar world. You'll want to treat it like a utility rather than a safe-for-kids app. If they use it as a songbook, it's brilliant; if they use it as a social network, you'll want to keep a closer eye on things.