Why doesn't Scout want Uncle Jack to tell Atticus why she beat up Francis, and what does it demonstrate about their relationship?

Scout doesn't want Jack to tell Atticus about this because, in her words, Atticus made her promise "not to let anything I heard about him make me mad." She doesn't want him to know that she has punched Francis because he said Atticus was a "n----r lover," and that he would be the "ruination of the family" (114). Jack, who had been upset at Scout for her altercation with Francis, then determined to see that Francis was punished instead. At this point, Scout begs him not to let Atticus know about it. This is one of several times Scout is upset by other kids' comments about her father, and this foreshadows the strain that the trial of Tom Robinson will put on their little family. It also demonstrates that she is deeply concerned about keeping promises to her father, who she respects. She demonstrates her concern for her father's opinion even more later in the chapter, when she eavesdrops as her uncle discusses Scout (without revealing why she hit Francis).