As someone who wanted the opportunity to see the Northern Lights, and could only go in school holidays, it was by chance that we were away over Christmas. The ship was beautifully decorated, and the staff led a Christmas service before we left Bergen.
We went with my 9 and 12 year old son (and 77 year old Grandma) We've done long distance train travel before so I wasn't worried about activities specifically aimed at the children. They loved the cabin bunks and the jacuzzi was a hit. The food was much more varied than I'd thought. Yes, lots fish-based, but presented in different ways and I like that fact that the boys ate the same as us.
We booked an off-shore excusion everyday (apart from Dec 25th when nothing ran, but we walked and explored instead). It was good to explore the towns by foot and to get off the boat for a while.
A queen sized, high bed with 2 bunks that flipped down from the wall. Plenty of storage. Clean, compact en-suite.