In Chapter 24 of To Kill a Mockingbird, Scout assumes that Mrs. Merriweather is talking about Mayella because Mayella and Bob Ewell, in Scout's mind, are the people in Maycomb who most notably have done something wrong and therefore are the only people who need to be forgiven.
Not to mention, prior to and following the incident with Tom and the trial, Mayella is someone who could use help. It is locally known in town that Mayella is like a mother to the Ewell children, having been forced into that parental role because Bob Ewell is useless. Mrs. Merriweather says, "Thing that church ought to do is help her lead a Christian life for those children from here on out." Scout assumes that "those children" are the Ewell children who Mayella takes care of because Bob is a useless and abusive father.
In Scout's mind, Helen Robinson doesn't need to be forgiven because she has committed no crime; obviously, neither has Tom. If anyone needs to be forgiven for some crime, it is Mayella and Bob Ewell. It is also logical for a reader to assume that Scout knows that some help and/or guidance from the community and the church might benefit the Ewell family. If anything, Helen needed to be apologized to.