What is the main idea of O.Henry's story, ''The Ransom of Red Chief''?

In short, the answer is "be careful what you ask for."  In O. Henry's story, the narrator and his friend, Bill Driscoll, have decided to kidnap the son of a prominent local citizen in hopes that his father will fork over money the pair need "to pull off a fraudulent town-lot scheme in Western Illinois."

However, when they manage to get the boy, (who calls himself "Red Chief") they get much more than they bargained for. The child fancies himself the subject of one of his favorite games, cowboys and Indians.  He fights them "like a welter-weight cinnamon bear." 

Despite their disparity in sizes, the boy manages to best the men at every turn:

"He put a red-hot boiled potato down my back," explained Bill, "and the mashed it with his foot"

Later, the narrator is awoken by horrific screams from his companion: 

I jumped up to see what the matter was. Red Chief was sitting on Bill's chest, with one hand twined in Bill's hair. In the other he had the sharp case-knife we used for slicing, bacon.

The men know they have made a terrible mistake and want to get "Red Chief" home as quickly as possible.  But the boy doesn't want to go.  He's having too much fun with his friends. 

So, again, be careful what you wish for....As the poet Robert Burns says, "The best laid schemes of mice and men/Often go astray."