Although Pierre has loving parents and a pretty good life, he has an insufferable habit of saying "I don't care" to just about everything. Pierre pours syrup on his hair, he flips upside down in the folding chair, and he won't "care" about anything, even when his mother plies him with Cream of Wheat or his father attempts to placate him by allowing him to fold the folding chair. (Incidentally, I never understood why either of these were used as some kind of enticing reward. Cream of Wheat is not very exciting to people under the age of 60, and I have folded a few folding chairs in my time and nothing magical ever happened. Maybe I was doing it wrong.)
His parents leave Pierre alone while they go to town (cf "Mother Love") and while they're gone, "A hungry lion paid a call / He looked Pierre right in the eye / And asked him if he'd like to die." The specter of imminent death should be a harsh reality check for the tyke, but no, he replies, naturally, "I don't care." So the lion ate Pierre.
Pierre's parents come home to find a lion in their son's bed who says "I don't care" in a suspiciously familiar voice. They take him to a doctor, who flips the lion upside down and shakes Pierre out. The lion offers Pierre a ride home if he would care to climb on him, and this time Pierre decides he does care. The lion stays on as a weekend guest, which must have been awkward since he'd already tried to eat his hosts' son, but maybe he brought a really good wine or something as a nice hostess gift and that smoothed things over.
Oh, this one's easy. "The moral of Pierre is, 'care'!"
Sendak, Maurice. Pierre: A Cautionary Tale. New York: Harper Collins, 1962.
Pierre: A Cautionary Tale in Five Chapters and a Prologue
(Carole King sings the story on YouTube here)