It is effective that Jem and Alexandra both ask who because it demonstrates that not everyone in Maycomb supports racism.
Atticus points out to Jem that not everyone on the jury was in a hurry to convict Tom Robinson.
"You might like to know that there was one fellow who took considerable wearing down- in the beginning he was rarin' for an outright acquittal."
"Who?" Jem was astonished. (ch 23)
Jem is surprised, because he assumed that everyone on the jury wanted to convict Tom Robinson. He is surprised to learn that there are some men who were willing to stand up for themselves, including the surprising figure of Mr. Cunningham.
In chapter 24, Miss Maudie points out that allowing Atticus to defend Tom demonstrates some townspeople’s faith in him and in the potential for change.
"Have you ever thought of it this way, Alexandra? Whether Maycomb knows it or not, we're paying the highest tribute we can pay a man. We trust him to do right. It's that simple." (ch 24)
Miss Maudie replies that there are a “handful of people in this town who say that fair play is not marked White Only” (ch 24), and who wanted Tom Robinson to get a real defense.
The fact that not everyone in Maycomb is in agreement with racism demonstrates that change can come, albeit slowly. Both Jem and Alexandra are surprised to learn that someone in the town cares, and they are not alone in the fight. It is not just children who care, but adults too.