I'd say Amsterdam. One of the main squares transforms into a Christmas village which has an ice rink and massive market. Some museums are open on. Christmas Day, including the Anne Frank house. Every year there's a spectacular light festival and an ice sculpture festival in the city and I've heard that this year there is going to be the world biggest production of the Swan Lake ballet from 23rd to 27th December at one of the theatres.
"For those who love classic ballet, here is anexceptional proposal, especially for the holidays. RAI Theater in Amsterdam will host the biggest Swan Lake Ballet between December 23rd and 27th. The innovation of this spectacle is that instead of 16 swans, the performance expands to 48 swans, which automatically renders the spectacle unique, taking into account that there has never been a similar performance of such size."
The mains reason I say Amsterdam is because there is more to do for children than Bruges.
Bruges is really magical and would get my vote, though having said that I have never been at Christmas time before. Also check out Ghent if you go to Bruges, you can get the train there and it is another fabulous city. Go to see the Van Eyck Ghent Alterpiece in the church there - so beautiful!
Another one for Amsterdam, sadly that you've been there already because this place is really worth visiting during Christmas (we were there two years ago and it was amazing!). Comparing to other countries many interesting places will be open there, besides they make such a beautiful Christmas village on one of the main squares, you really feel there like in a fairytale From our experience I can recommend Estonia too, Tallin is another nice place for spending Christmas holidays. There are many interesting places to see and visit greentourbaltic.com/tourcats/tallinn/ and like in case of Amsterdam not all of them will be closed during that time. There also will be amazing Christmas market, I really recommend to visit it, also many interesting events and concerts devoted to Christmas too. Lithuania and Latvia can be good choices too, also France, Paris in this case is one of the most romantic places ever