There is nothing harder to estimate than a writer's time, nothing harder to keep track of. There are moments—moments of sustained creation—when his time is fairly valuable; and there are hours and hours when a writer's time isn't worth the paper he is not writing anything on.
Anyone who writes down to children is simply wasting his time. You have to write up, not down. Children are demanding. They are the most attentive, curious, eager, observant, sensitive, quick, and generally congenial readers on earth.... Children are game for anything. I throw them hard words and they backhand them across the net.