Writers understand what L'Engle means by "the book that wants to be written." We have a narrative that runs in our heads that will either drive us crazy or bring us joy. The only way to stay sane is to write that story and get it out of our heads.
Children, even those who are old enough to differentiate between reality and fantasy, are more willing to suspend disbelief than most adults. They delight in unfamiliar creatures and worlds. They cross over to fantasy and back to reality with ease and without confusion. So, if your work befuddles grownups, perhaps you need a more sophisticated audience. L'Engle did and children have delighted in her work for years.