Some questions from Act IV, Scene 3.
what strange power does the king possess? what qualities seem to make him a good king? and... why do you think macduff comments about macbeths lack of children?
I assume that in this question you are asking about Edward, king of England, and not about the king of Scotland.
Edward is said in this scene to have the power to cure the sick just by putting a gold coin around their necks and saying prayers over them. From this, I would say that the quality of caring is what makes him a good king. He cares enough about his people to heal the afflictions of even the "little people."
As far as Macbeth's lack of children, I would speculate that this shows that Macbeth is not able to be a father to his country (as a king should be) any more than he is able to father children. It may also be a way of saying that he is not a true man.