Thy Dungeon man 3!
stupid witch. The scrapbook clearly shows 4 kids. But 4 isn’t the answer. Neither is 3, 2, or 1, apparently. I guess I just need to sit here and wait for hrwiki.org to update.
Update: Looks like a glitch or something. I restarted the game, and this time it worked perfectly. Technology. Gotta love it.