Bill and Sam are con men. Sam, the narrator, says:
Bill and me had a joint capital of about six hundred dollars, and we needed just two thousand dollars more to pull off a fraudulent town-lot scheme in Western Illinois with.
Evidently they need the two thousand dollars to buy a large piece of land, or at least put a down payment on it, and subdivide it into town lots. They probably intend to keep selling the same lots over and over again. No doubt they would accept small down payments and only give the buyers receipts, telling them they would get the deeds when the lots were paid for in full. These are called "land contract" sales.
The "fraudulent town-lot scheme" characterizes these men as con men and not kidnappers or baby sitters. They get into trouble by venturing into an area of criminal activity with which they are totally unfamiliar. The moral of this story might be expressed with the old saying "The cobbler should stick to his last." They should have stuck to the kinds of swindles they knew about.
Another moral for this story, of course, is "Crime does not pay." Not only are Bill and Sam forced to give Red Chief's father two hundred and fifty dollars to take the kid off their hands, but their situation at the beginning of the story shows that they have gained only six hundred dollars with all their dishonest dealings and all their running around in the Midwest. They are always on the lam, and the opening of the story finds them sleeping on the ground and cooking their meals over a campfire.