Intro to literary theory recommendations for a precocious 15 year old

My daughter is taking honors English and AP European history and this has led to some conversations with her during long dog walks about Marxism, and then critical theory and ideology, and she's been asking questions along more theory lines about the books she's reading. (Right now that's The Great Gatsby.)

I was wondering if people here have any recommendations of starter texts for her to start exploring theory more? I have my old textbooks and anthologies from college, but they're too difficult for her right now. Is there a broad overview or introduction, either to literary theory general or to specific major schools, that would work better for a smart 15 year old who is, nonetheless, still a 15 year old?