Congrats! Disney cruises are so much fun!
My family actually did a 4-night Halloween on the High Seas a few weeks ago. For our cruise they had the Halloween festivities on the day that we were in port in Nassau, Bahamas. This is also the day that you will be able to meet some of the characters in their Halloween attire. They had 2 different party times, like a dance party type thing where the Trick-or-Treating takes place. They will give you a small bag in your stateroom, I'm assuming one per child but we only had 1 child out of the 2 staterooms we had, so we had 1 bag.
We had gone to the second of the two parties and there were a ton of people dressed up in actual costumes. The actual Trick-or-Treating, while obviously a bit busy, was, in my opinion, well organized. There were cast members standing at the midship stairs instructing people where the stations were and some of the cast members were announcing how many were on each deck so that you knew whether you hit them all or not. I believe that that stations were on deck 4, 3, and 2, with a couple on each floor. The stations are marked with a sign that says like Mickey's Mouse-querade party or something similar so you know where they are. You also can't miss them since cast members, in Halloween costumes, are standing in front of large cauldrons of candy.
The candy was very plentiful. Some stations had treat bags already made up that they handed out while others just gave out handfuls of candy. We finished, after hitting practically every station, with a very full bag and a very happy child. Even now when we ask him if he's going to go Trick-or-Treating for Halloween he says that he wants to go back and do it on the
Disney cruise!