Like so many books, hush combines elements of several genres. It's young adult literature because our main girl is a young adult herself, so it deals with issues common to young adults—think: school, peer groups, and identity formation. Most importantly, though, it's written for young adult readers. So no dictionary required.
Since the story centers around the specific experiences of one family, and how their dynamics and relationships work and change over the course of the novel, hush is a family drama, too. The Greens are like a wind chime, and when one moves, they all go into motion. Plus, since they're in witness protection, they spend a of time together—it's hard to make friends when you have a hard time remembering your new name.