First, figure out what's the best part of the costume. Then get in close and fill the frame with the parts you've decided are best. If you're shooting a child or a group of children, bend down low to kid's-eye level. Don't shoot from adult level down on these little ghouls - you'll trivialize them. Or if you're limber enough, bend down to below kid's-eye level or even lie down for the shot.
Nothing makes a monster more imposing than looking up at the scary countenance.
If you're shooting two monsters, get them as close together as possible and, again, get close enough to fill the frame with the best parts of their costumes. People tend to drift apart when there's a camera pointed at them. That way in future years everyone will be able to identify the little devil behind the mask, and this year you may be able to use the picture for your Xmas card. The same tips go for portraits of adults in costume, only you don't have to get down so low to be at eye-level. But don't forget the possibility of bending low or even lying down to make the monster look taller and scarier.