I don't have a family setup watch so I can't check for sure. These pages from Apple:
https://support.apple.com/en-us/HT211768 https://support.apple.com/guide/watc...0a51fb/watchos
Don't say anything about Music and whether it is supported. But on a normal watch you can use Watch app / Music / Add Music to add songs (I believe that you can add albums or playlists, but if the album has only one song in it, this would work). It would have to be in your music library on the phone, so if you don't use Apple Music, you should be able to either buy the song with iTunes or add the music on a computer in iTunes and sync to your iPhone.
See
https://support.apple.com/guide/watc...798d11/watchos
What they don't mention is whether this is different with a family setup watch.
I use AutoSleep on my watch. Again, not a family setup watch, but AutoSleep does tell you on the watch what the sleep stats are. Tat one costs, though, so I would suggest a "free" (ad-supported) app that is also excellent called Sleep++. As with Autosleep , it automatically detects sleep, though you can manually tell it when you "turn off the lights" so it can track how long it takes to fall asleep. (Of course, it should be stated that sleep tracking is just guesstimation without an EEG to actually track sleep cycles. These watch apps - all of them - are only guessing based on movement and heart rate, so don't get too worked up over the stats.)