I don't actually know, I've only ever had 1 trick-or-treater at my house though I've lived her for 5 years. We only have a couple close neighbors and none of them have kids. We only had Bitsy, Charlie, and Buster then and I don't remember how they reacted to that one kid. That was our first year here.

I know the cats would all hide, because they do that whenever someone new comes over. Buster would run and hide too, because he is afraid of all new people. I'd guess that for Bitsy, Charlie, Nacey, and Nova it would depend on the costume. If some little girl came to the door dressed as a princess they'd all probably run over to her to be pet. However if someone came to the house dressed as Darth Vader or something they'd probably either run away or bark out of fear.

Interesting thread.