The following information comes from Jennifer Maurer, School Library Consultant for the State Library of Oregon. Jen posted this on the edWeb.net School Library Network discussion board. Great information, thank you Jen!
Most free eBooks that come with no restrictions are those that are available in the public domain, which means they are no longer under copyright or they never were. If they never were under copyright, they are often self published. Most new to new-ish and copyrighted materials are not available for free, or they come with some restrictions. Even with digital versions of books, the authors, illustrators, and publishers need to get paid by someone in order to make a living or profit.
As … noted, Epic! offers free eBooks to elementary students. The catch is that to access the books at home, parents have to pay to subscribe.
You can check what your local public library offers. In that case, the public library subscribes on behalf of its patrons. Popular platforms include OverDrive (with the Libby app), Cloud Library, and for kids only, some offer Tumblebooks.
International Children’s Digital Library is a grant-funded project that makes children’s books in many languages available online at no cost.
Book Bub is a site that tracks temporary deals for free or inexpensive ebooks. You can sign up to receive a daily email. I’m pretty sure there’s a category for children’s books.
And for classics and other public domain offerings, there are many sites, including Project Gutenberg and Open Library (which has some copyrighted material; not sure how that is okay). This article lists other sites…
By the way, Sync offers 2 free audiobooks per week to download during the summer. The books are aimed at ages 13 and over.