Actually, the only character whose behavior is NOT really surprising at the conclusion of this ironic story is that of the kidnapped boy. From the beginning, Johnny is described as a hellion; he is captured while chucking rocks at a kitten, and throughout the story the behavior of this "captive" is fiendish. It is hardly surprising, therefore, when he resists being "reunited" with his loving family.
The kidnappers, on the other hand, begin by assuming they have a perfect plan--kidnapping the young child of a prominent community member and demanding a ransom for his return. As events unfold, however, the two "desperate" men become so desperate to be rid of the little scamp that they abandon all hope of getting money and seriously consider agreeing to the father's request that THEY pay HIM to take the boy back!
The father, in turn, does not play the part of a concerned or anxious parent. Rather, he enjoys the respite Johnny's "kidnapping" provides, and at the end, only promises to give the kidnappers a head start in escaping the boy, who so enjoys his time with the kidnappers that he is likely to attempt to find them and continue the game.