The short exchange between Lady Macduff and her son in act 4, scene 2 humanizes these two figures, especially the intelligent young boy. He is not simply an abstraction to us. Like Duncan and Banquo, he becomes a flesh and blood character we get to know a little. Unlike Duncan and Banquo, however, he is murdered onstage. To have Macbeth's assassins stab a child in front of the audience's eyes brings home, as words cannot, the horror of what Macbeth has become. There is no reason for Macbeth to have this child killed: it will be years before he can pose any realistic threat to the throne. Having him murdered shows that Macbeth has become a despot, whose actions make him utterly unfit to rule Scotland. Macbeth's thought before he kills Duncan, that one murder will lead to another until he is drowning in blood, has come true.
Further, the boy's awareness, shown through his dialogue with his mother, that the world is a corrupt and evil place shows the ill effects of Macbeth's reign. His mother says that liars and traitors must be hanged. The boy states to his mother that the liars and traitors of the world outnumber the honest enough to turn the tables:
there are liars and swearers enough to beat the honest men and hang up them.
Macbeth's initial murder of Duncan has had effects that reverberate across Scotland, unleashing evil to the point that even children are aware of it.