Violet tells Caroline not to bother with the housework, as only she knows how to do it properly. Among housekeepers of the old south especially those familiar with hoodoo there are practices, such as using herbal washes on the floors and windows, laying lines of brick dust in doorways, and marking the corners of rooms with ammonia (or urine) to protect from evil.
Herding Caroline to make the circle and "dig herself in". There's no apparent reason, since they have issues with the whole believing thing, not getting people in circles, and apparently the spell doesn't require the victim to trick himself/herself. Then it hits: making up a "protective circle" is the ultimate test of faith, where the victims both consciously and unconsciously accept to believe, and show others that they do. And if you say outloud you believe in protection, you must believein its opposite too...
It's the ultimate test of faith because instead of using some tools to make a hole and some rope to run away (A mundane solution) she tried to protect herself with magic. She put her faith in some magical protection, instead of herself, "contaminating" her spirit with superstition and belief.
The plight of Ben/Luke is a lot more grim when you realize the twist ending. A poor, unsuspecting guy, young and with his whole life ahead of him, happens to land two seemingly sweet old folks as his clients. And then, they trick him into believing in hoodoo, perform a terrifying ritual on him, and trap him in the paralyzed body of an old man until he dies. And he appears to just be aware enough to recognize what Caroline's in for, and to beg her to rescue him.
Heck, the entire movie is Fridge Horror, if you think about it. Pretty much every single thing Luke and Violet say to Caroline serves as some sort of foreshadowing to their plans for her.
What the villains did to the children went beyond simple murder: their spirits weren't just displaced when the ritual was performed; they were forced into Justify and Cecile's bodies. The children, probably barely concious, were lynched and burned by their own parents. And no one was ever any the wiser. They at least had the small mercy of being rendered essentially brain-dead. Not like what happened to Cecile and Justify's later victims.
But the parents died in a murder-suicide. Could it be the dad figured it out and spared his wife the realization, since there was no way to fix it?