Another good one for little kids is dipping one of their feet in white paint.
Then have them press the foot on a black piece of construction paper
Turn it upside down and you have a ghost
Then (when dry) take a permanent marker and you draw on the mouth and eyes of the ghost. Looks great when done. was a hit with the parents. http://pinterest.com/pin/259379259758350181/

One I read for little ones is to take a black piece of construction paper and put it in the bottom of a plastic tray....then put a few drops of white paint and a ping pong ball or a marble, close the lid and have the toddler shake and move the marble.
YOu end up with a spider web drawing. Let it dry and then glue on a plastic spider or a paper spider and voila - spider web