In order to evaluate progess, there must be an established reference point for comparison. Have students produce untutored drawings before they receive instruction.
Drawing with Children; A Creative Method for Adult Beginners, Too
by Mona Brookes
Founded on the belief that any child can learn to draw realistic pictures using her "alphabet of shapes" while in a noncompetitive environment, Mona Brookes' easy-to-follow, lesson-by-lesson approach to drawing has yielded astounding results with children of all ages.