Aunt Alexandra is referring to Calpurnia in this quote. Since Alexandra is now there to help with the children, she thinks Calpurnia is not needed. Alexandra would prefer that Cal leaves for other reasons that she will not say out loud. Alexandra thinks people should associate with people in the same social class. She reveals this elitism when she scolds Scout for being friends with Walter Cunningham Jr.
Alexandra thinks Atticus is too lenient with the children. Atticus is much more liberal and open-minded than Alexandra. When Alexandra arrives in Maycomb, Scout notes that she fits right in with Maycomb's history of acknowledging social classes and hierarchies. It becomes clearer, the longer Alexandra stays, that she has good intentions but her adherence to Maycomb's social traditions (which includes residual racism) and strict ideas about how the children should behave are antithetical to Atticus' style of parenting. Atticus believes in being honest with the children. He is a paradigm of righteous behavior, so he would of course find no problem with Scout going to visit Calpurnia at her house. This is what Alexandra objects to in this quote, but Alexandra is objecting to Atticus' parenting in general.
Atticus defends Cal and says she is part of the family:
“Besides, I don’t think the children’ve suffered one bit from her having brought them up. If anything, she’s been harder on them in some ways than a mother would have been… she’s never let them get away with anything, she’s never indulged them the way most colored nurses do. She tried to bring them up according to her lights, and Cal’s lights are pretty good—and another thing, the children love her.”