If you are skeptical about audio books, please read this article that explains how current research shows that people's brains engage with audio books much the same as the written page.
https://www.discovermagazine.com/mind/audiobooks-or-reading-to-our-brains-it-doesnt-matter)
https://www.mensaforkids.org/achieve/excellence-in-reading/
https://www.commonsensemedia.org/lists/50-books-all-kids-should-read-before-theyre-12
http://www.ala.org/alsc/publications-resources/book-lists
https://www.goodreads.com/list/tag/elementary
https://www.greatschools.org/gk/articles/books-for-kids-grades-k-5/
Older Kids and Adults:
http://www.opensourceshakespeare.org/
Young Adults:
https://www.booksie.com/genre/young-adult
https://www.classicly.com/young-readers
Kids Books:
http://en.childrenslibrary.org/
https://etc.usf.edu/lit2go/readability/flesch_kincaid_grade_level/ (by grade level)
https://www.storylineonline.net/
http://www.openculture.com/freeaudiobooks
https://sites.google.com/site/freechildrensbooks/home/oz-books
http://www.aaronshep.com/stories/classic.html
http://allenmorgan.com/free-1.HTM
https://www.americanfolklore.net/index.html
https://americanliterature.com/short-stories-for-children
https://archive.org/details/iacl
https://www.childrensbooksonline.org/library.htm
http://hca.gilead.org.il/ (Hans Christian Anderson stories)
http://www.childrenslibrary.org/icdl/SimpleSearchCategory?ilang=English
https://kids-pages.com/stories.htm?count=15
https://lightupyourbrain.com/stories/
https://www.longlongtimeago.com/
http://www.planetozkids.com/index.html
http://www.angelfire.com/ok2/ordinal/
https://www.barefootbooks.com/kids/animated-videos/
Something for Everyone:
https://books.google.com/?hl=en