Later, my wife figured it out: The other day she had sung the "old man is snoring" song, but replaced "old man" with "grandpa". In other words...
It's raining, it's pouring, grandpa is snoring
Bumped his head on the bed, now he can't get up in the morning
Okay, now it's not so mysterious as to how our toddler got an image of grandpa falling down and sleeping, is it?
But here's why skepticism is a win-win: Since meaning is something humans invent, I can take whatever personal meaning from this I want to. My folks haven't gotten to see the boys in a few weeks, and I really ought to set up a time for us to get together. I managed to avoid the ominous fear that a more credulous person might have felt, but I also get a useful reminder to keep in touch with family. Huzzah!