What is the irony in the story of "The Ransom of Red Chief" By O. Henry?

Anyone who has ever been around a mischievous or hard-to-discipline child can appreciate the irony in this story.  The two bumbling kidnappers, Bill Driscoll and the narrator, are going to try to get $2000 in ransom for a fraud scheme up in Illinois.  They select the son of a well-to-do mortgage broker, who they expect will pay every dime of the $2000 that they need.  Their first hint that this might not go well should have been when they pick up the kid while he's throwing rocks at a kitten in the street, after which he gets Bill in the eye with a piece of brick.  What follows is one headache after another trying to manage this hard to manage kid--until finally, in the ultimate humourous irony, Driscoll and the narrator give up.  The kid's father not only won't pay them ransom, but helpfully offers to take him off their hands only if THEY pay HIM $250, which they do at once, as they absolutely cannot take another minute with the little thug.