Arvind Gupta (who I'm not familiar with; a google search tells me he's a well known science educator in India.) put this great little collection of free books together. http://gyanpedia.in/tft/Resources/books_e.htm
There looks to be a nice science section and if you scroll almost all the way down there's an entire section of Isaac Asimov PDFs that would be a great resource for teaching kids.
Just because I chuckled when I saw it, I particularly wanted to point out this book called "How Babies are made" it's one I'm well familiar with and read it quite a bit as a kid. The pictures still crack me up. LOL I'm pretty sure it's out of print now. It's a decent book for teaching the subject of reproduction in a simple way, without a morality component, for small kids. I'm sure there are plenty of other books out there these days but this one does the job and has the advantage of being free. http://gyanpedia.in/tft/Resources/books/howbabiesaremade.pdf