I need a fun science experiment to do with kids.?

You could try the baking soda volcano, or make dry ice fog by putting dry ice in a container of hot water. I don't think first or second graders will give a crap about the zodiac. However, the older children will probably enjoy learning about it. http://chemistry.about.com/cs/howtos/ht/buildavolcano.htm

How about 'does astrology really work?' Cut the horoscopes out of an obscure paper none of them would have read that morning, and take off the month part of it so you can't tell which one goes with who. Then have the kids try to pick which one fits them best. It's no better than chance. When they read it in the newspaper with the dates attached, they try to fit whatever it says to their life, and then think it works - but without the dates, it doesn't work at all. They are all general enough to fit everyone in most cases. Then you can talk about the ecliptic, the path of the Sun in the sky, and how all the dates are off by at least a month anyway and there are actually 13 constellations in the zodiac.