As a sometimes-writer-of-picture books who is often asked to pare back the story to the basic essentials, I found this post most appropriate. As a former teacher, I think it is true that we often under-estimate the abilities of children. Many, publishers included, have been dumbing down storylines to the point that practically any picture book might be considered an easy-to-read text. Yet, I believe kids crave more and are quite capable of dealing with extended lengths. Have a read of a most informative article: http://blogs.publishersweekly.com/blogs/shelftalker/?p=16276.