I have noticed that playing certain factions it seems to be hard to get quality heirs, especially when playing the danes, the english, and the almos. I have found 2 ways to avoid situation altogether.
1. Throw your toothless glutton at armies of bad guys. Nitzche(sp) is right, whatever doesn't kill him, will give him better virtues. Do this to your heirs to be as well.
2. Ignore it. If I have bad royals, I usually end up with rockstar generals. I mean a general with 8*, 8 acumen, who just so happens to be a saint. Well, pretend he is king. He probably does more for the kingdom then your worthless king anyways ... or you could start a rebellion and hope he leads the charge.