Above all, Shirley Jackson is a great storyteller. In this short story, the element of plot structure, dialogue and irony are important. If one reads this short story knowing a little about Jackson and the types of stories she writes (such as The Lottery), one is immediately suspicious at the outset that there is something more to this "Charles" than meets the eye.
I watched him go off the first morning
with the older girl next door, seeing clearly that an era of my life was ended, my sweetvoiced nursery-school tot replaced by a long-trousered, swaggering character who forgot to stop at the corner and wave good-bye to me.
Right away, the reader gets suspicious. This foreshadowing builds as the story progresses. As this humorous story unfolds, we see the son, Laurie, telling stories about how bad Charles is in school, when simultaneously Laurie is being reprimanded by his parents for his own poor table manners, speech, disrespect, etc. The dialogue is fast-paced and amusing. The story is almost like a play.
One can almost figure out how the story is going to end -- that there IS no Charles, and that Laurie is Charles. So the irony slowly builds and the climax of the story is at the end, when this reality is eventually revealed.
Read the story and analysis here on enotes.