It is clear that Atticus means for his bad news to be only heard by Scout and his trusted friends--sister Alexandra, Miss Maudie and Calpurnia. Alexandra and, particularly, Miss Maudie must have understood that the news of Tom's death was meant for their ears only, and that the events of Tom's death would be spread around town soon enough. Atticus wants to be sure that Tom's wife, Helen, learned of her husband's death from him and not second-hand. Most importantly, they all knew that the supposedly "devout ladies" of the Missionary Circle would have had little true sympathy for Tom. They may have pretended to have shown some concern--Scout had already decided that several of the women were "born hypocrites"--but considering the racist remarks they had already made about some of the Negroes in Maycomb, Alexandra must have realized that it was best to continue serving refreshments as if nothing had happened. Several of the women had already made it clear that since Tom was charged with rape, "there's no lady safe in her bed these nights." Mrs. Merriweather believed Atticus's defense of Tom was "misguided," and all it "did was stir 'em up." It is actually Maudie who demands that Scout "Stop that shaking" before telling Scout and Alexandra to show "not a sign" to the other women. Alexandra, the host, simply followed Maudie's lead, and the "gentle hum" of the women soon resumed.
Probably better the way she did it, telling the ladies would only bring up more racist conversation.