I did this on a 9x12 sheet of paper, and pre-folded it into 6 equal squares for the kids. On their paint palettes I put ROYGBV paints, along with white and black. (I used liquid tempera, it mixes the easier!) The kids drew two parallel wavy lines across the top three squares and again across the bottom three squares. Inside each "wave" they painted the original color ROYGBV in spectrum order.
Above each wave (original color) they mixed a shade of that color, and below each color they mixed a tint of that color. I must say that the hardest part of this project was making sure that all shades didn't turn out pitch black (they dipped just the tippy-top of their brush into the black and mixed).
*Keep in mind that the three examples I have here are pretty darn good. MANY MANY of them weren't able to stay in the lines, etc...so of course this is adaptable for probably up to third grade! I didn't focus on that so much as the paint mixing.*
A VOILA! A great lesson on spectrum order, tints, and shades :)