Let's see: Fictional characters? Check. Real-life setting? Check. Time period from the past? Check. We think it's safe to say that we're dealing with historical fiction here.
Cassie and her family are fictional characters (although inspired by the experiences of Mildred D. Taylor's own father and grandfather). They live in a real setting (rural Mississippi), and the action takes place in the 1930s (see "Setting" for more details on this). The social and cultural context the Logans deal with is very real, also: the racism and oppression that black people lived under in the South during this time.
The narrator is a kid, and a kid who learns a major lesson by the end of the novel. It deals with universal themes of childhood, like growing up; not understanding the grown ups; and trying to figure out your place in the world. And, like all good young adult books, it has a major coming-of-age plot.
Roll of Thunder, Hear My Cry is a sort of transitional coming-of-age story, meaning that we don't see Cassie totally grow up. Remember: she's only nine years old, and we only see one year of her young life. Cassie learns some pretty tough lessons for someone so young, like how to pick her battles and how to navigate the dangerous waters of black-white relationships. But, she's still got a long way to go before she even hits those teenage years.