In a large liquid measuring cup, stir together the water and cornstarch until there are no clumps. Divide the mixture into a muffin tin or individual bowls.
Place one color of your chalk into a zip-top bag. Using your rolling pin or mallet, smash the chalk until you end up with a fine powder. Stir this into one container of the chalk mixture until you have your desired color. Repeat this with the remaining chalk until you have all your preferred colors.
Allow your child to paint on sidewalks or driveways with chalk and paintbrushes or even their little hands.