Here are some other quick ideas that are perfect last-minute solutions:
Mummy: White clothing with surgical gauze or toilet paper wrapped around the child's face and body makes a classic mummy.
Dice (great for siblings or friends): Get two large square boxes and cut out holes for the children's bodies and arms. Paint the boxes white and add black painted dots.
Dominos or Playing Cards: A black poster board with white dots is as good a domino as any. Attach string at each side at the top so that the child can wear the costume like a sandwich board. Fashion a playing card out of white poster board. Let your child help paint the Queen of Hearts or the King of Spades onto the front and a fun pattern on the back.
Cat/Mouse/Dog:It doesn't get much easier than this. Brown, gray, or black clothing, eyeliner for whiskers and a sock filled with paper for the tail gets you a Halloween pet. To make ears, purchase a headband and cut and tape construction ears to the top. For a dog, include a toy bone; a cat can carry a stuffed mouse, and a mouse can carry a hunk of cheese.
So toss those elaborate plans aside, and postpone the expense of a store-bought costume for another year! If all else fails, you can always dig out a white sheet, grab a pair of scissors, and start cutting!