I was thinking child friendly, so using NFC tags isn't such a bad idea - plus I have a few hundred spare ones in nice plastic casings. I'd also just store IDs on them, and either have the media files preloaded (as seems to be the case here), or have it download it from my media server on first use.

Other thing I want (which they don't do) is the ability of resuming playback at the same position, even when putting it into a different player - that's one reason I still have some audio cassettes for the kids. No other medium I'm aware of does that kind of easy state saving. My idea there is to have the tags locked in the player in a way that gives me enough time to write the position if the user tries to remove it.