One of the themes of "The Flowers" is that a young child is not shocked or frightened or even sickened at coming across a dead body. The girl even steps on the body without much of a reaction.
A movie that might echo the same theme is "River's Edge," about a high school student who kills his girlfriend and then shows her body to their friends. The friends want to either protect the boy or turn him in to the police, but no one is really shocked that a girl is dead.
Another similar movie might be "Stand by Me," but that's more about a group of boys' curiosity about a dead body.
There are a lot of movies out there that are centered around the persecution that black people received, and even some that deal specifically with lynchings, white violence, and the injustice surrounding these issues. Try the movie "A Time to Kill" (also a novel by John Grisham), or "Mississippi Burning" if you are looking for movies that cover the injustice of a lot of the violence against blacks in the 50's-70's. Then, for a book, I would suggest "Mississippi Trial, 1955" for a great book that covers the brutal murder of a Emmett Till that got unfair treatment in the courts. It is an easy read, based on true events, and very interesting. It is also similar to "The Flowers" because it is told from the perspective of a young boy. If you want to add more Alice Walker to your list, you could try "The Color Purple", set in a bit different time period but about the struggle of black people to survive in a harsh world. This has also been made into a movie.
Those are a few suggestions, and I hope that they help.