Why does Big Ma take the children to Berrys?

Mama wants the children to understand just what kind of people the Wallaces are..... the things they are capable of doing. She is also reaffirming the reasons she does not want them going to the Wallace store.

A still form lay there staring at us with glittering eyes. The face had no nose, and the head no hair; the skin was scarred, burned, and the lips were wizened black, like charcoal. As the wheezing sound echoed from the opening that was a mouth, Mama said, “Say good morning to Mrs. Berry’s husband, children.”