If you have the Watercolor Workbook, it takes you through step by step on how
to do trees w/ shading. My suggestion would be to use this ref and the next
time they come in take them through it step by step as a class- have done
this w/ 5th's and it does take them a whole class. You'll need to provide
some palettes for them to mix/ dilute on.
Hope this helps
ps of course, have them create more than one tree, maybe 3, one each in fore,
mid, backgrounds so there is a chang ein intensity of color from atmospheric
just my thoughts.....