One of my munchkins is pretty close to fully reading, the other is a while off yet, so it would be nice if the kit was applicable to both skill ranges.
((My kids are 3 and 4. My three year old's favorite toys are her magnifying glass and flashlight :-))
I don't know about any Science kits but for our youngins they loved bug catchers. They catch them and then put them in this bucket with a maze and a magnified glass (it's built in and safe). They love catching ants, beetles, etc. and just watching them. We then looked them up on the internet and presented some basic facts about them and let them go.
There are several "Magic School Bus" science kits on the market. I just discovered them on Amazon. They seem to be very well reviewed. I have a 5 yr old and I am going to get him the one about germs.
Try a magnets set, one with wands, horseshoe, and magnetic balls. They often come with guides, or you can look through Usborne books catalog to see what sort of "Science with Magnets" books they offer to supplement for ideas. Your library may also have easy-to-do books on projects with magnets.
Also, sometimes books have a lot of great projects which can be done with things from around the house.
Oh just get some bubbles.
A prism. It makes "rainbows."
Make homemade play-doh.
Use baking soda and vinegar, you can color it with food coloring.
Make a "volcano."
When you combine baking soda with vinegar, it foams up.
Your kids are real young. You will need to read any directions for them?
What exactly are the ages of your kids?
The dinosaur excavation kits (complete with goggles and chisels) were popular at our house. The bones didn't stand up and stay together as well as I'd hoped, after the extraction, so we reused them in the sandbox for further dino digs.
Check this out! it's full of science experiments for preschoolers!
http://www.childcarelounge.com/activity/science-experimen...
