Well the whole process went surprisingly smoothly. With clear direction, and others following the guidelines each of the boys did a great job of drawing on their canvas. Then with some help, limited paint and a controlled environment they painted within their own drawing.
One week on, we returned and re-drew our drawing to re-define the picture and then painted it with a shiny vanish.
These are now displayed proudly above each of the boys beds!
Sometimes I worry about setting too many limits on our kids but then I see them create something wonderful like this that they love and enjoyed doing. This helped me to remember that there is a time for directed play as well as free play.