The most practical/cheap option would probably be an old phone with parental locks and access to a streaming service. It'd be much more functional and cheaper than any dedicated screenless device, and it gives a kid much more autonomy. It's understandable to want to protect kids from predatory algorithms, but music streaming services are probably the least predatory out of all the "big" apps. Also they never stopped making cheap mp3 players, if you want to have complete control over their music discovery