You drop the kids off. I have not had kids in Adventure Ocean, but when I was walking laps, I saw the kids in the program (through the window) and they always looked like they were having a lot of fun.
I was on a cruise over Halloween. A lot of people dressed up, not just kids. I don't know about Trick-or-Treating. I did not bring candy, but if everyone had, a cruise ship would be a sweet place to Trick-or-Treat.
I'm sure Adventure Ocean plans activities for the kids. There was a Halloween party, but I don't recall kids there.