The narrator of that chapter is speaking in a Southern American dialect throughout the novel. She substitutes "a" for the correct "an" and also for "of" later in the same passage:
Taking care a white babies, that's what I do, along with all the cooking and the cleaning. I done raised seventeen kids in my lifetime. I know how to get them babies to sleep, stop crying, and go in the toilet bowl before they mamas even get out a bed in the morning.