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.
Painting will be very helpful for people with post-traumatic syndrome, or those who are constantly in a stress state. The fact that such activity calms the amygdale, i.e. the area of the brain that is responsible for generating emotions. When you focus on coloring pictures, you give your brain a chance to rest and relax.