オズのまほうつかい 10歳までに読みたい世界名作

This is an abridged version of The Wizard of Oz by L. Frank Baum, translated into Japanese for kids. It's short (about 150 pages), the font size is gigantic, and all kanji have furigana (but as with most children's literature, there is not as much kanji as one might wish, so parsing long strings of hiragana can get a little tiresome).

Very easy to read due to the simple sentences and familiar story, probably the easiest in this series of abridged world classics. Be aware that the story is somewhat different from the movie, but the book in English is in the public domain so you can easily find it and check it if you want to be sure you've understood it correctly in Japanese.

There's a surprising amount of world literature available translated into Japanese for kids, so if you like and are familiar with classic books like these, you may find them good material for beginner reading starting at upper N4.