Film set in past (maybe with Charlize Theron)

I need help finding a movie that is set in the past. The only scene I remember is a pretty blond woman (who appears to have a mental illness) has a breakdown. She takes the children out in the rain and beats them (with a belt?) under a tree. The field hands stare in horror until someone stops her. I think the actress is Charlize Theron but I could be mistaken.