This is an easy project for little ones. Have them color on construction paper with a crayon that is the same color as the construction paper. Red paper + red crayon, black paper + black crayon. We used black crayon.
You want them to color really hard, so it might work better for them to color on the floor so they can use their big arm muscles.
Then, spray the paper with water and watch the colors change.
Little ones like to spray water (and the color-changing action is pretty cool, too).