In my humble opinion, as both a long time artist and amateur astronomer, the best way to teach your kiddos about how art can interact with science, especially Astronomy, is to use pictures of the constellations and the planets then relate some of the oral traditions and myths (also "art" by the way) that make up the stories behind those celestial bodies. You can compare the art and stories of what the ancient Greeks had to say about certain constellations to what the Chinese or American Indian traditions were.
If you really want to blow their socks off, hold up a Curious George book and tell them the same guy who created that book (H. A. Rey) was also interested in the stories of the night sky and wrote (and illustrated) his own books on the subject: "The Stars" and "Find a Constellation". (It would be great if you bought/borrowed a copy so you could also hold it up). Even though Starry Night is one of my favorite paintings and Van Gogh a favorite artist of mine, talking about the constellations and telling about H. A. Rey will have more impact on third graders. You could also have the children create their own constellations (based on a real star map) and make up their own stories about them.
