“Goodnight Moon” does two things right away: It sets up a world and then it subverts its own rules even as it follows them. It works like a sonata of sorts, but, like a good version of the form, it does not follow a wholly predictable structure. Many children’s books do, particularly for this age, as kids love repetition and the books supply it. They often end as we expect, with a circling back to the start, and a fun twist. This is satisfying but it can be forgettable. Kids - people - also love depth and surprise, and “Goodnight Moon” offers both.
Haven’t read Goodnight Moon in ages…at 4 and 7, my kids protest whenever I suggest it. We’re currently powering our way through the third Harry Potter book, which, though I enjoy Potter, is no Goodnight Moon.
Update: How Goodnight Moon overcame bad initial reviews and became a word-of-mouth bestseller.