Here are four of the best websites for written and audio stories for children! They are mostly beginner-elementary level but the first website also offers higher levels with translations as well as audio.
https://www.wasabi-jpn.com/japanese-lessons/fairy-tales-and-short-stories-with-easy-japanese/ This is probably my favourite site; these stories also provide JLPT levels in case you'd like more of a challenge! There is also an audio option for each story with various speeds. The actual reading part has tiny Hiragana above each Kanji so you don't need to panic when faced with those awkward Kanji ;-)
http://life.ou.edu/stories/ Each story has a breakdown of the words and phrases used so it's great for grammar and vocabulary building!
http://hukumusume.com/douwa/betu/ This site is completely in Japanese with no translations so it's great if you feel like figuring it out yourself. :-)
https://www.youtube.com/user/fantajikan/videos Audio! Yay! Close your eyes and listen. :-)
Hukumusume has an English language version, but it is much smaller than the main site. It has all of the folktales that include an English translation. The main site has even more children's stories written in Japanese only (or with non-English tanslations)
Here is a link to the English language site. Some stories also have audio.
Thanks for that. :) Duolingo has stories (under Labs) for four languages but not Japanese. It would be cool if the team could create Japanese stories here too.
Damn ! Thanks a lot. I am building a course by using the classic "Language Learning Secrets Revealed" by Peter D Campbell.