Merging two families to a single apple music account
I am getting married soon, and me and my fiancee both have Apple Music, with with our children. If she cancels Apple Music, and joins my family plan, I understand that her playlists, and her children's playlists, will go away. What steps can be taken to prevent that? Can they share their playlists with me, then cancel? In other words, what happens to a shared playlist when the owner of the playlist cancels Apple Music?
Or, is there a simpler way to "merge" these two Apple Music family plans?