Yep. I think this is the standard path for parents today. Kids even get to keep some Angry Birds and PBS Kids cartoon from their formative iPad years.

The camera on my Pixel 5a shit the bed, so I just replaced it with the 8a. Because everything else about it still works fine, I keep it around as a home Wi-Fi device. Most of what I use it for is playing spotify. I have a waterproof Bluetooth speaker for in the shower and I'm not even bothering teaching the new phone to connect to it.

If OP has an old phone as a backup, yeah I can see it fulfilling this need really easily. There are even pretty thorough parental control options (on Android at least) to have a separate user profile for a kid and have control of that user supervised through another device.

If OP doesn't have an old phone for this use case, probably the best bet is to either buy a used, older phone and set that stuff up or just find an old used iPod or similar. If something like spotify is essential to this plan, newer iPod touch models running iOS 15 support spotify. Otherwise, you can find a pretty basic mp3 player for under $50, but you'll need to source music files yourself for that ;)

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.