So that´s the situation:
I´m the owner of a Spotify Premium Family Account. I´have one Echo dot and one Echo - perfect linked to Spotify, no problem.
Then my two boys have a Echo Dot and are members of my premium family account, their two Echo dots are linked with their Spotify-Family-Accounts, I´ve chosen the right location and set spotify and not amazon music per default.
But: they can just listen to a few songs, their Echo dots don´t find that much music on spotify.