Most of the above tools will report readability for formally printed texts -- NoveList and TeachingBooks are both very consistent at providing that kind of support for both fiction and non-fiction. When needing to check the readability of other kinds of texts -- a newspaper article, a website, etc. -- consider trying the free tool below:
The Curriculum Center in the Lower level of JFK Library has thousands of children's books.
Many publishers provide book discussion guides on their websites too.
It is worth checking the Education databases for articles as well, along with bibliography books in the Zs in the Curriculum Center's teacher materials.
Bookmark these blogs, set them as home pages, add their RSS feed to a reader like Feedly, like them on Facebook -- keep up with what's out there in children's and YA books!