To give the animal shapes a rounded form and not so flat look, we shaded. First, the students practice shading on a separate paper by making a 3-step value scale for dark, medium, and light. Then they practiced shading from dark to light several times. I remind them shading and coloring are not the same thing. Now we're ready to shade.
Shading the outer edges dark and going lighter in the middle is how I usually teach it. We use color pencils and crayons.