The irony is that Charles does not exist, and the family has been judging Laurie the entire time.
Irony in literature is “the technique of indicating an intention or attitude opposed to what is actually stated” (“eNotes Guide to Literary Terms”). It is when what happens is different from what is expected. The irony in this story is that the horrible boy Charles in kindergarten with Laurie is really Laurie.
Laurie is a troublemaker. His mother should be able to see this, but she is too wrapped up in her complicated life to notice. She has a new baby who draws away most of her focus. From the beginning of the story, Laurie’s bad behavior is described.
At lunch he 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.
When Laurie starts kindergarten, he describes a terrible boy who always acts up and gets in trouble. The entire family seems to enjoy listening to the little boy’s mishaps, even though their own child is no angel. The irony is that they are so judgmental of Charles and ignore Laurie’s misbehavior.
Laurie’s mother can’t attend the first P.T.A. meeting, but she goes to the second one, excited to meet Charles’s mother. Laurie's parents gleefully discuss meeting the mother of the horrible boy.
“Invite her over for a cup of tea after the meeting,” he said. “I want to get a look at her.”
“If only she’s there,” I said prayerfully.
“She’ll be there,” my husband said. “I don’t see how they could hold a P.T.A. meeting without Charles’s mother.”
They are being very judgmental. At the meeting, Laurie's mother meets her son's teacher, who tells her Laurie “had a little trouble adjusting, the first week or so.” She then tells her that there is no Charles in the class.
Although the story ends there, the implication is clear: Laurie made up Charles. All the behaviors he attributed to Charles were his own. He wanted to tell his parents what was happening, but did not know how or feared the repercussions.