A friend of mine has a daughter who is too young for a smart phone, but he still wants her to be able to listen to music on the go. I found the following devices using spotify. Have you used any of these or do you have any alternative suggestions?
https://www.tunepat.com/spotify-music-tips/portable-spotify-music-player.html
Edit: found what I was looking for. https://bemighty.com/
Could alternatively use a data-only card if you want to allow streaming.
You can then limit the data used a couple ways - through your account, or with a local app or phone config (and some of these apps use a pin)
You could also use an app like No Root Firewall to limit network access (and Android can enforce an always-on VPN, preventing network access if the VPN isn’t running). It too can be locked with a pin.
Just some ideas if you want to allow streaming with specific apps (no root permits you to block network for all apps, with exceptions). I use it with a prefilter that blocks a bunch of social media and ad domains.