When I teach value to my 5th graders we shade spheres using colored pencils.
as an introduction I turn off all the lights in my room and shine a
flashlight on a basketball from different angles. They notice the areas that
do not face the flashlight are the darkest and the spot where the light hits
is the lightest value. I also point out the values in between of course. I
will also show them some reproductions with a variety of values.
Hope this helps.