if you have not booked for halloween yet, i do believe you are going to be out of luck, however i did cancel my regular reservation yesterday for halloween, but it fills up fast...you will probably have to wait for a cancellation like mine (i went into a group ressie instead)
anyways...trick-or-treating at the fort is awesome and the kids absolutely love it...it (in my opinion) is much safer and quieter than where i live and there are a ton of kids around, along with adults, all dressed up...most of the campers will either be out in front of their sites or have a bowl of candy for the kids to take some of...
my son had his first try last year and loved every second of it