We spotted these beautiful coloring books while browsing at an art store. They're a great way to introduce children to classics and a big upgrade from your average character-based coloring book. Here is a page from the Norman Rockwell book.
Your kids can color in translucent pages of Tiffany stained glass meant to be torn out and placed on a window. They can also color the works of M.C. Escher, Edward Gorey, or Impressionist Masterpieces from the Musee d'Orsay.
Check out your local art stores in the childrens' section for these. We also found a whole bunch available on Amazon.