The Catcher in the Rye
Phoebe may be a fourth-grader, but she still has a lot to teach Holden. First, she asks him important and incisive questions (as in, "You don't like anything. Name one thing you like a lot" [22.38]). And she helps him out by covering for him when their mom comes home, lending him money, and putting the red hunting hat on his head when it starts raining.
Phoebe understands Holden in a nuanced and sophisticated way, and though she's younger than he is, she's adept at guiding him through his toughest times (like when she puts her arm around his shoulder while he cries). When she brings a suitcase because she wants to run away with Holden, it seems a lot like she wants to take care of him as much as she wants to run away. She also teaches her brother a lesson, if unintentionally; when he watches her reach for the gold ring on the carousel, he's realizes that growing up is inevitable, and maybe it’s not so bad anyway.
Phoebe Caulfield: the greatest ten-year-old mentor ever.