I am also on the 9/3 sailing and I called to ask. They didn't know much but the CM claimed that there will be a Halloween dress up trick-or-treat night, although she did not know how they would do it socially distanced and which night it would be.
I plan to just throw the kids costumes in the luggage just in case but I am not holding my breath. If anything, I expect they will have bags pre-filled with candy one night that they can hand out to kids, but not the same Halloween festivities they usually have. Since they split the fireworks into 2 nights, I can see them doing the same for Halloween. Maybe Night 2 the early dining does fireworks on top deck and late dining does Halloween indoors, and then Night 3 is reversed. Or maybe they do a Halloween dress up night during dinner and pass out candy right at your table.
I guess we will find out in 2 weeks!