As I am most often hosted by libraries, I see a lot of science books for preschoolers. I kid you not most of them are not good. Some are outdated, other incorrect, and some are misleading.
One entire series stands out Let's Read and Learn.
A number of titles as a question - and I love that. Science is about exploring and that must start with a question. I wonder...? How does that...? What happens if...?
Many of these titles are available at your local library - check them out next time you and your preschooler have a question