What is great art and how can I go about teaching my child art appreciation?

"It is beginning to dawn upon us that Art is great only in proportion to the greatness of the idea that it expresses; while what we ask of the execution, the technique, is that it shall be adequate to the inspiring idea." Charlotte Mason

Guernica by Picasso

"We recognise that the power of appreciating art and of producing to some extent an interpretation of what one sees is as universal as intelligence, imagination, nay, speech, the power of producing words. But there must be knowledge and, in the first place, not the technical knowledge of how to produce, but some reverent knowledge of what has been produced; that is, children should learn pictures, line by line, group by group, by reading, not books, but pictures themselves. A friendly picture-dealer supplies us with half a dozen beautiful little reproductions of the work of some single artist, term by term. After a short story of the artist's life and a few sympathetic words about his trees or his skies, his river-paths or his figures, the little pictures are studied one at a time; that is, children learn, not merely to see a picture but to look at it, taking in every detail. Then the picture is turned over and the children tell what they have seen,––a dog driving a flock of sheep along a road but nobody with the dog. Ah, there is a boy lying down by the stream drinking. It is morning as you can see by the light so the sheep are being driven to pasture, and so on; nothing is left out, the discarded plough, the crooked birch, the clouds beautiful in form and threatening rain, there is enough for half an hour's talk and memory in this little reproduction of a great picture and the children will know it wherever they see it, whether a signed proof, a copy in oils, or the original itself in one of our galleries." Charlotte Mason