A.B.
Have you tried the book, How Nature Works (by David Burnie)? Although it says it is for kids grade 4 and up, we recently ordered it, and there's a lot that younger kids can do.
If that doesn't fit your needs, the link below has a pile of science books for kids ranging from age 3-9:
http://www.letteroftheweek.com/science_of_the_week.html