Why is the ending of the story ironic, or the opposite of what you would expect? In the short story 'Charles'

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 ... The irony is that they are so judgmental of Charles and ignore Laurie's misbehavior.