Having worked a few Christmas cruises, they are can be very busy, with many large extended family groups on board.
Shorter (cheaper) cruises have so many extra kids running about that additional kids club staff are brought on just for these few weeks! (i’ve met a few who normally work in schools and then work on ships during school holidays - not sure how they survive that!)
That said, the decorations are beautiful, festive energy is contagious, and the entire ship just seems happier.