I did suggest the Magic School bus series though...I remember watching that when I was in college (Hey, I had a little too much free time on my hands...LOL!)...I think that is a good place to start.
I think these are really great books for 4-6 year olds. I also recommend Highlight magazine for fun science in general for kids.
Here are some of the books my kids like. There are mostly written in story form so capture the interest of the little kids:
Floating in Space (from the Let's-Read-and-Find-Out-Science series) by Franklin Branley
The Best Book of Spaceships (Scholastic series)
And for 6-8 year olds (or older):
If You Decide to Go to the Moon by Faith McNulty - written as though the child is taking a trip to the moon, discusses the experience of the astronauts, describes space, how the moon was formed, overall good quality.
Our Solar System by Seymour Simon (Scholastic Books) - neat book, a little complex for very little kiddos, though my 7-year-old loves to have it read to him. Full of facts.
I'm always on the look out for good quality science books for them. If I find anymore, I'll update here.
The one by Hawking sounds interesting. I wonder if any other astronomers/physicists/scientists have written books for kids.