Prepare the poster board for the preschoolers first. Cut out 6-by-6-inch pieces of poster board. Measure each square out using a ruler and then cut these pieces out with scissors. The small size is ideal for preschool children.
Place an even layer of old newspapers on the worktable or on the floor where the children will be working. Place the poster board pieces and the crayons on the work surface.
Tell the children to color the entire surface of their poster board using as many crayons as they want. Color along with them and keep an eye on their progress. Patterns in solid stripes or swirls of colors work best, but allow the children to create the patterns they prefer.
Squeeze out a couple of tablespoons of black tempera paint into small bowls for the children. Show them how you want them to paint over the crayon designs they just created. Let the preschool children start painting, while supervising them closely, to avoid large spills or upsets. Help children who are having a difficult time getting the paint to go on smoothly.
Allow all of the poster boards to dry completely for at least 30 minutes in a safe, clean place. Mark each child's name on the back of the poster board as you collect them.
Place the dried poster boards back on the work surface and give each child a wooden toothpick. Illustrate how to use the scratchboard to the preschoolers. Take your toothpick and scratch out lines and shapes to reveal the colorful crayon underneath. Encourage the children to do the same.