My opinion of STORY GENIUS is generally good, especially the section about your character's misbelief. I think the weakness is that it purports to get into "brain science" when it really doesn't. That part is basically boiled down to "our brains are wired to respond to certain structural elements and that's why structure is important."
MG novels can be character-driven; e.g., many contemporary stories lean literary and don't have a lot of action but may still have a lot going on emotionally. Even if your book is primarily plot-driven, it still needs a defined character arc for MG. STORY GENIUS is useful and worth reading, although I favor both SAVE THE CAT WRITES A NOVEL and THE ANATOMY OF STORY over it.