The father realises that there is an inner evil in his son which needs controlling. There is frequent use of onomatopoeia such as when he describes his son as an "ogre", despite his size. The boy still wants revenge on his parent, believing the latter thinks nothing of him. Since the boy has thoughts of wanting revenge, the punishment viewed in this light seems to be justified.
The father hopes in time, his son will "understand" the need for discipline, and this is expressed through the words "but dare not ruin the lessons you should learn". This occasion is also tough for the father because he is hiding his inner core of uncertainty and suffering "behind that mask", after "slapping" the boy. The father is not repent; he is a man who follows absolute values. He doesn't seem to comprehend that mercy is more powerful than judging to the letter of the law.
The use of contrast, such as the boy's slack behaviour and his father's firmness, helps add depth to the relationship between the boy and the parent. Other examples of contrast include the boy's diminutive nature and powerful influence over his father, the vulnerability of his character and cunning, evil and shrewd mind. It is incredible to understand that this three year old possesses such qualities.