You know, I never really noticed it, but an awful lot of classic children's books are about dead pets. Not exactly the cheeriest of subjects.
And really, when you think about it, a lot of what kids read and watch (and I'm talking the "good" stuff that adults want them to read and watch), is really reather morbid. "A Christmas Carol" really isn't terribly cheerful; Frosty and Rudolph are kinda freaky; Charlie Brown is maudlin. Where's all the "innocence" kids are supposed to be about? Oh, that's right, that's just sentimentalist nostalgia from bitter middle-age pundits.