Children have a neat way of looking at things. Children do not see things as adults do. I may see a person that has a disability and look at them in a different way than I would someone who was healthy. I may be afraid of them or be more cautious around them than I would someone who was fine. A child may see the same person and see them as different but not treat them any differently than they would their friends.
Children are so much more innocent and much less judgmental than adults.
I think this is a positive exercise because it helps to see what details children think are important or which ones they may overlook. This has a lot to do with development because the more features your picture had the older you were. The fewer features your picture had the younger you were. Some features like ears or clothes a child may forget or may not think about when they are thinking of how to draw themselves. So, the idea is the more developed the self-drawing is the more developed the child is.