Can I set a maximum wobie box volume in the wobie fav app?

You can't currently set a maximum wobie box volume in the app.

You can lower the volume in Spotify for the current session. That level will stay in place while the wobie box remains on.

Recommended volume: The WHO recommends no more than 75 decibels for children's ears. 

Spotify playlists can vary in loudness. That's down to Spotify, not the wobie box.