Put them in a pan or box and cover them with dry, hot sand. After the sand cools, gently remove the leaves and smooth them with a warm iron. Dip each leaf in clear varnish and let it dry. This is an easy project, great any age. When you are finished with the prints you'll see that the colors come from the chlorophyll in the leaves.
Parental supervision is recommended because it involves beating the leaves with a hammer.
Printing Take a nice fall walk outside. Pick any up leaves that you like. Different colors and shapes make the best designs. I've used red maple, ferns, maple, daisy leaves. Try to stick to leaves, as flower petals do not work very well.