I am very excited to have just received our new science curriculum, God's Design for Science.  This should last for a while as it is intended for grades 1-8.  I can eventually use it for all three girls at the same time.  And the best thing about this package, is that it is totally biblically accurate.  It teaches Creation, and not just "creation" but a young earth created a little over 6,000 years ago and created in 6 actual 24 hour days.  It disucess the theory of evolution but shows why the theory is obviously wrong.  If you are interested in learning more, check out the link.  It has very high reviews by homeschoolers, science teachers, and the like.