What does Mr. Avery mean by "they ridin again"? Why does Mama make the kids go to bed? What does she mean when she says Stacey does help her, more than she think?

Mr. Abery is referring to the Night Riders, members of the KKK. Mama makes the children go to bed, so they won't be around if the men come to the Logan home.

Stacey helped Mama on the farm, in the house, and with his siblings..... he was there for him when she needed him, but she didn't want him to put himself in any danger.