Perhaps a study in the parts and varieties of trees would help with the
detail problems. They could practice on a seperate peice of paper before
moving on to the final composition. I always tell my students to remember
when they were in preschool, and they made "lollipop" trees to try and get
them into thinking that they have moved WAY past this point. (Aren't I bad!)
They usually buy into the ploy, and avoid those lollipops!