The picture should be large without small details, images must be recognizable, kind and interesting, for kids contour lines should be thick, it hides some errors made by child, the outlines of the pictures should be smooth, without corners, difficult bends.
Believe it or not, but the painting will help to improve intellectual skills. It stimulates those brain regions that increase the concentration and attention. It will help solve organizational problems. It seems strange that such activity in general is able somehow to train the skill of time management, but it's true. The fact is that for structuring the information meets the frontal lobe of the brain, and the painting of small and intricate details intensifies its work. The more colors you use, the more complex designs there will be, the brain will work better.