The hardest sort of story to write is not autobiographical, experimental / post-whatever or stories about faraway or fictional societies. It's children's books. I've come to this conclusion again and again, especially after reading something like Kiki's Delivery Service and comparing it to any number of other, more "adult" or even "literary" books in my collection.
It takes uncommon skill and discipline to pare out, to leave out, to strip down. A children's book is a distillation of many such acts. You are expected to keep things on a level suited to younger readers, and yet retain the kind of graceful complexity that will draw them back in again and again, possibly even as adults. To my mind, the whole thing is a little like having children for real: you need to just dive in and let it happen, and let the process of doing it change you.
Writing a bona-fide children's book is one of those things I've had kicking around in the back of my mind for a long time. I suspect reading Brave Story (yes, I've still got that on my list of things to do) will provide some perspective on how to make that happen on a large scale, where Kiki was remarkably trim. (Memo to self: pick up untranslated original for Kiki.)