My son has been asking for a music player for awhile now - we try to minimize overall screen time and did not want to get him a phone/etc. After a lot of research, I settled on getting him a Mighty 3 for his recent 9th birthday - it has been a MASSIVE hit, he loves it and takes it everywhere - in the house, on the road, it's always with him.
My one caveat, which is the reason for the 4 star - while syncing with Spotify in general goes very quickly, actually syncing individual playlists has been hit-or-miss. I put together 3-4 playlists with 10-15 tracks each, then added in a Spotify-generated playlist that was larger (about 70 tracks). On multiple attempts - sitting with the Mighty plugged in, immediately adjacent to the phone, actively watching the phone/not letting it go into sleep mode - it never fully synced all of the tracks from any of the playlists. This is in the same room as the router, the phone had full battery, etc - my son was still thrilled with the music he had, but he has also requested a few tracks that haven't synced, and there's no way to tell which tracks DID complete sync, just the number ("9/15tracks synced, etc").
