On the analogy of the famous apple,—“there ain’t going to be no” preface, “not nohow.” Children do not read prefaces, so anything of a prefatory nature that might interest them is put at the beginning of chapter one.
As for the grown-ups the story is not written for grown-ups, and if they want to know why it begins with such a gruesome first chapter, let them ask the children. Children like the horrors first and the end all bright. Many grown-ups like the tragedy at the end. But perhaps the children are right and the grown-ups are standing on their heads. Besides they can skip the first chapter; it is only a prologue.