We discovered them last weekend.  Nice Aunt Liz got him a sticker book.  And we were off.  Since then we have bought several more sticker books.  Two nights ago, he woke up at 1am crying “stickers, stickers, stickers”.  I guess he was dreaming about them.  (Last night he woke up at 1am crying “waterpark, waterpark, waterpark”.  This waking up briefly around 1am has kind of been a thing for the last week.)