Christmas Day most everything is closed, except for Palazzo Strozzi. They can go visit the exhibit there on Olafur Eliasson. On Christmas Eve everything is still open but generally museums close by 6pm so they can still plan to do things on the eve.
As for a special Christmas lunch in particular, all restaurants should be open and might offer the option of a special menu. Is there something in particular they want to try or a budget they are looking for? Obviously the high end restaurants might make the lunch extra special, I highly recommend you get a reservation for them ahead of time.
