Because Scout blames Walter Jr. for getting her in trouble with her new teacher, Miss Caroline, on the first day of first grade, she allows her flying fists to settle the dispute. Scout's quick temper and her urge to settle arguments by fighting are eventually quelled somewhat as she grows older, but she is still angry with Walter after Jem breaks up their fight on the schoolyard. Jem takes pity on Walter, in part because he is smaller than Scout, but also because
"Our daddy's a friend of your daddy's. Scout here, she's crazy--"
On their way to the Finch's house for lunch, "Jem made pleasant conversation" with Walter, "cordially" comparing stories about Boo Radley as they passed his house.
By the time we reached our front steps Walter had forgotten he was a Cunningham.
Once inside, Atticus and Walter "talked together like two men, to the wonderment of Jem and me." Jem's behavior toward Walter stems from his understanding of the Cunningham family. He knows they are poor but honest, and he sees that the puny Walter is underfed and no match for Scout, who is big for her age. Jem is old enough to realize that inviting Walter home for lunch is a fair way of repaying him for Scout's unhospitable manners, and though the two boys will never be close friends, Jem walks with Walter back to school while Calpurnia finishes her scolding of Scout.
It depends on which Walter Cunningham you mean (the father or the son). The father is a poor farmer who pays their father by trade. Scout treats him very nicely; however, Mr. Cunningham is embarassed that he cannot afford to just pay Atticus. When the mob goes to try to take Tom Robinson at the jail, it is Mr. Cunningham who realizes they need to leave because Scout points him out and he feels silly that they are acting the way they are in front of children. If you are referring to the son, Scout is upset with Walter because he got her in trouble at school. Scout tried to intervene to prevent the teacher from offering him money because she knew he wouldn't take it. Scout ends up fighting him and gets in trouble. Atticus makes her invite him over to dinner and she is flabbergasted at the way he eats his dinner. Since he is used to not having much, he soaks up all he can of their rich food. She starts to say something, but is stopped by her father because he is trying to teach her manners. Hope this helps!