Accessories and a little face paint often do more for a costume than the costume itself. They are also where most of the evening's small disasters happen, so it pays to keep things simple.
Test face paint first
Use products made for children's skin and patch test a small dot on the inner arm a day or two before. Stop immediately if the skin looks red or irritated.
Keep the design to two or three elements
A cat nose and whiskers, a spider web on one cheek, or a few skeleton lines around the eyes read beautifully in photos and take two minutes. Elaborate full-face designs smudge within the hour.
Set it and carry a fix
A light dusting of translucent powder helps paint stay put. Bring a few wipes for the inevitable touch-up, and take everything off before bed with a gentle cleanser.
Choose accessories that stay on
Headbands with a soft inner band, clip-on wings, and capes that tie at the front survive an evening of running. Anything balanced on the head without a clip will end up carried.
Mind the small parts
With younger siblings around, avoid loose beads, small jewels, and anything that detaches easily. Soft props are safer in a crowd of excited children.
Add light
A glow stick bracelet or a light-up wand doubles as decoration and visibility after dark. It is the single easiest addition to any costume.