[m-i-l]

I got a Taf Toys "My First MP3 Player" a few years back. Parents could load a USB stick with MP3s on any PC (including Linux), then plug the stick into a secure compartment on the device (requiring a screwdriver for access, like the battery compartment). Children then had buttons for play, stop, next, previous, volume up, volume down. Only minor issues/features were that the maximum volume was pretty low (presumably to make it safe for babies), and it would switch itself off if no buttons were pressed for a short while (again presumably so it would be safe to leave unattended). Was going to post a link, but it seems it is no longer available.