Dd wanted to do this last year at age 6 also -- what a coincidence. We happened to be hanging out with another family in which the mom is absolutely passionate about astronomy (even named her firstborn after an astronaut). We did lots of cool stuff together -- she had them walking around in big circle in the basement to illustrate planet rotation vs. orbit, made a comet out of dry ice, dirt, ammonia, corn syrup, etc., had them drop junk into a pan full of flour and cocoa powder to illustrate how impact craters form.
Things we did: make Milky Way (glitter on black paper), illustrate variety of of galaxy structures (probably got this from NASA for kids website -- we had a major computer crash this year, so I lost all my files on where I got what and website addresses); made solar system out of Sculpey and papier mache that was correct in relative size (our friends hung notes on their basement walls to show how far apart the planets would be -- we didn't have space for that in our house), made star cookies to help remember which stars are hot and which stars are cool (recipe from NASA, I think). Music tie-in -- Twinkle-twinkle Little Star, of course, as well as some space CDs I found at the library. Handwriting practice was space jokes. I made up math games, like a number line that had a space theme -- we rolled a die to add and subtract, going back and forth to Pluto and the Sun, etc. Did constellation dot to dots. Art tie-in -- did our own interpretation of Van Gogh's Starry Night.
Books of experiments and activities: Janice Van Cleave's Astrnomy for Every Kid, of course. Young Astronomer (ack -- I didn't write down authors!). How the Universe Works. Wishing on a Star.
There's a Montessori catalog that had gobs of cool stuff -- Michael Olaf. We got a solar system floor puzzle and a solar system mobile from there. I forget their website addy -- sorry. Hobby Lobby also has some cool looking stuff in their science craft dept.
Also, don't overlook the websites that have unit study info. I'll probably think of more stuff we did later. It was really fun.
