Eight nesting dolls sit on a mantle, all sisters. One is different from the rest: Lilly doesn't open. She discovers THE SURPRISE INSIDE, that she is special too, the only one who can save the human family's holiday surprise #PIT2PUB #pb
I am terrible at this, so take my suggestions lightly. Is there some other word you could use at the end of the pitch besides surprise (which is already in the name of the book)?
Good luck!