Calpurnia is more than a housekeeper and cook. She is part of the family. She is a teacher to Scout and Jem. She is a caring but strict disciplinarian. Calpurnia treats Scout and Jem as she would her own children.
Atticus trusts Calpurnia. He supports her and gives her the authority she needs to discipline the children. When Scout is upset with Calpurnia for correcting her manners concerning the Cunningham boy, Atticus sides with Calpurnia. He knows that Calpurnia loves Scout and Jem as she would her very own children. This shows that Atticus is not prejudiced in any way. Also, his children do not judge Calpurnia based on her skin color.
Atticus does not worry over his children when he is at work. He knows Calpurnia is watching over them carefully. In fact, Atticus trusts that Calpurnia will not only feed the children, but she acts in their mother's absence. In a sense, Calpurnia is like the children's mother. Calpurnia has raised the children since their mother died four years ago. Atticus trusts whole heartedly in Calpurnia's judgment:
She has helped to raise Jem and Scout since their mother's death four years ago. Like Atticus, Calpurnia is a strict but loving teacher, particularly in regard to Scout, whose enthusiasm sometimes makes her thoughtless.
Calpurnia even takes the children to church with her. She is not worried about what the children will think. She is comfortable sharing her personal life with the children. Indeed, Calpurnia is family. She understands her role. She loves the children. The children love her as well. Again, she treats them as if they were her own children.