crayon shavings
waxed paper
hot iron

Tear a long (about 16") sheet of waxed paper and fold it in half width wise.
Have your kids sprinkle crayon shavings onto the one half of the wax paper. The more you use, the more colorful it will be. Fold the clean half of the paper over the arrangement and carefully lay in on the ironing board. Or on top of a surface covered with a towel. Have an adult gently press the iron over the entire sheet of paper until the two halves are stuck together.

After they have cooled, hang the pictures in a window that receives direct sunlight