When I find a nonfiction book that they can understand, it is a big deal! We recently found Bajo El Agua by TwoCan publishers at the library. I searched on amazon and found they have a series of these informational books in Spanish. Here is a link to the search I did.
It has information explained in pretty simple language. I also loved the back page with questions. It was great for a discussion and to see that my kids really did understand the book.
Do you have any other informational books or series in Spanish that you use or like?