I went on that same exact cruise a couple of years ago. They held a New Years celebration with the typical party favors and free champagne. You don't have to dress up if you don't want to. Back when we went it was cold on the pool deck where the held the countdown, so be sure to bring a jacket, although with the freak warm weather everyone seems to be having you may get lucky. Other then the party favors the cruise was no different then any other sailing the Fantasy takes. Your kids should have a great time, I saw many of them running around throwing streamers at everyone.