What is the irony of the witches' prophecies for Banquo in light of the prophecy that Macbeth "shalt be king hereafter"?

The witches words to Macbeth suggest that he shall have a long reign as king, especially considering the word "hereafter".  However, their prophecy to Banquo is that he will be the father of kings.  We quickly learn that Banquo already has a son.  It would be against tradition for Macbeth's throne to go to Banquo's son man years down the road - one would assume that Macbeth would have his own children at the that time to pass his crown to.  Therefore, Macbeth's propehecy is ironic because it would seem to be contradictory in light of Banquo's.