The humor in the story "Charles" can be relatively blatant for some readers (like students), while others (like parents) may find it lacking humor.
The story's humor lies in the fantastical life of the narrator's son Laurie. The fact that her son changes into a "swaggering character who forgot to stop at the corner and wave good-bye" can bring laughter to many readers. Picturing a kindergartner "swagger" is quite funny.
Outside of that, the behavior which Laurie shows is funny. Upon returning from his first day of school, Laurie
spoke insolently to his father, spilled his baby sister’s milk, and remarked that his teacher said we were not to take the name of the Lord in vain.
This behavior is far from the expected behavior readers would expect from a previously "sweet-voiced nursery-school tot." Later, some readers may find Laurie's own laughter humorous.
“Look down,” Laurie said. “Look at my thumb. Gee, you’re dumb.” He began to laugh insanely.
In the end, the fact that Laurie made up Charles to hide his own rambunctious behavior is the most humorous aspect of the story itself. While some may feel sorry for the narrator, others may figure that Laurie was simply "being a boy" and find it funny that he acted as he did.