While you may have to react faster, the secret is to decide on the best part of the costume before you shoot, then get in close enough to fill the frame with this part. Chances are, you'll have to use strobe to get enough light, but often you'll get a better-lit picture if you just use available light. Of course, if you plan to use available light, once again we suggest you use a fast film - ISO 400 or faster. We really have two different types of situations here. Second, pictures you take from outside a neighbor's door when you follow around after your own baby goblin.
Kids come and go mighty fast.
Here's one time when flash on camera can come in handy. It's best if you have two people in the house. One to answer the door. The other to take the picture from behind so that the photo includes both the person answering the door and that porchful of ghastly visitors. Tell the ghouls they'll have to grimace and groan before treats are dispersed - then snap the shutter while they're howling.