I cannot find the reference, but in one of her non-fiction books,
Madeleine L'Engle wrote that if she had a truly difficult concept to write about that it would be in a "children's" book. George MacDonald's Curdie books were children's books, but the level of writing in them would be suitable for a college literature class. YA is a very respectable neighborhood.

Just dropping in - I enjoy your Darkness & Light trilogy and your blogging.
Ann in GA