In Chapter 1, I think it is, what happened to Grandma's mailbox in A Long Way from Chicago?
The incident about which you are asking actually occurs in the second chapter, entitled "The Mouse in the Milk." Grandma Dowdel's mailbox is "blown to smithereens" by a cherry bomb.
It had been a quiet, uneventful day, and after nightfall, Grandma had been dozing in her rocking chair while Joey worked on a puzzle and Mary Alice "was sulking somewhere," having been told to come inside while she had been playing with her jumprope. Joey hears the sound of a horse stopping momentarily outside, then galloping off at great speed. There is a sudden flash of light that fills the bay window, and an explosion shakes the house. Grandma, Mary Alice, and Joey rush to the front door, and see that where the mailbox had been, there is now only a stump, the post that had held the box up. The mailbox itself "is gone - in several directions." The three can hear the sound of a piece of the mailbox sliding down the shingles of the roof, bouncing off the gutter, and falling into the bushes.
Grandma has only one thing to say - "Cowgills." She knows that the vandalism has been caused by the notorious Cowgill brothers, and begins to hatch a plot to teach them a lesson.