I think the best way for children to learn art is for them to have lots of opportunities to do it and for them to have a lot of freedom when they are making art. It is great to give a kid a crayon and a piece of paper, but it is even better if you can give them a box of crayons and markers and a stack of colored paper.
Kids like options and they have active imaginations. Letting them pursue any path that they want goes a long way to helping them understand art, design and aesthetics. Letting them experiment is a lot more effective than trying to drive the information into their brains.