If your going on the Halloween cruise you can dress up in just about any costume you like.
There is a 70's Disco Night, they put on a show using people from the audience and they do dress them up like the Village People. You can take your Disco clothes and join in.
Kids dress up but not so much as characters, mostly little girls, as ppiew has stated, in the "Princess" dresses. We have seem some little boys in Buzz costumes at dinner.
BTY- Halloween aboard one of the Disney ships is really a HOOT!!!!