A few thoughts that I ran with, until the terrible, terrible things I was doing wound up depressing me too much, and I moved back to catholics.
- Try to marry genius wives (or wives with other nice traits). The extra children make it that much easier to pick up good traits in your children.
- Have at least a few of those wives be high-intrigue, ambitious, etc, then foster the non-genius sons to them. Decent chance, the kid will be taken care of before he gets old enough to give you any decadence.
- Other than that, take the hit as necessary until your heir inherits and you can imprison for free. Give your brothers jail cells for their 16th birthdays. Having good stats can help out your decadence a fair bit, actually.
- note that if you imprison the wie for some reason while she's pregnant (say, because you just caught her assassinating one of your other kids) and she gives birth in prison, the kid starts life in prison, and can be left there without tyranny (unless you want that one) (note that with the health penalty, you might not be able to leave them there *long*.