How do the children contribute to the theme of the poem?

The speaker describes a situation involving a child demonstrating his/her intelligence in a humorous fashion. The child receives a one dollar bill from his/her father and swaps it for "two shiny quarters." This is the beginning of a series of swaps that leave the child with considerably less money. The humor resides in the fact that the child thinks increasing the amount of coins in possession means the numerical value of his/her money increases. For example, the child trades the two quarters for three dimes, three dimes for four nickels, and four nickels for five pennies. His/her dad learns about the swaps, responding with reddened cheeks and a look of disapproval. Yet the child mistakes his demeanor for pride in his smartness.