Thanks to this post by 40hz
, I discovered a treasure trove of free children's ebooks available in your choice of a plethora of formats, including Kindle.
Almost all of them are very old and in the public domain. Some great classics in there, along with a lot I never heard of.
When I was a kid, I loved old books. Give me some ancient reading textbook (at least 3 grades above me) or an antique bird watcher's guide and I'd be much happier than with the newer stuff the other kids were reading. I devoured antique fairytale books and things you can no longer find in most public libraries. I think this site is about as close as you are going to get to reproducing that kind of experience for your children, in ebook form.
And while it is not listed in the Young Readers section, you can start with this one: http://manybooks.net...hec2769027690-8.html
I remember eye balling that book on the top shelf for months, then finally making the librarian climb a ladder to get it down for me. I had the privilege of being able to take home and read a first edition copy of that book when I was about 9 years old. I loved the book so much.
Also made the librarian go fetch this one, right after (an earlier book by the same author): http://manybooks.net...hec2510225102-8.html
Again, I got to take home a first edition copy.