I used to do a lesson using Georgia O'Keeffe flower paintings. They
drew from a plastic flower first. Then blow up close and draw. Paint
with Tempera using complementary colors. Show how to mix complements to
darken and dull colors. Use complements in shadows and background.
Also start with the darkest color and use this color to outline entire
flower. Paint up to the outline but don't cover. Very effective and
easy to succeed technique for that age group.