Thought I'd take the liberty to direct you to a few great articles on photography, since you'll want to get awesome shots of the beggers...
How to photograph a ghost (easy photoshop instructions here.)
Photojojo's Ghoulish Guide to scary pics
20 Tips from BHG (I found these quite informative.)
Shutterfly's Tips for Trick or Treaters
Neat Lighting Tricks here (including turning your camera upside down and using your flash, creating a neat effect with the light reflecting upward--kinda cool!)
Hope you get a few great shots of your favorite little monsters. Trick or Treat!!