A vassal declares war on the heir. What to do

I have recently made my heir the ruler of a small county I recently conquered. Problem is, one of my other vassals suddenly declared war on my heir over his county! Most uncool.

I've not yet given my heir any other titles, and since it is a small and recently conquered county, he can barely raise any levy of his own. Defeat is therefore imminent for him. I thought about imprisoning the aggressor, but it seems challenging my heir is not a justified reason to do so. Nor can I revoke his title.

As I see it, I have a few options:

1. Give control of more powerful counties to my heir, thus making his
   powerbase stronger
2. Let them duke it out for themselves, and plot against the aggressor
3. Imprison/assinate/revoke the title of the aggressor anyway

I don't really like the third option, even if it seems the most natural. The reason being, that the bishops for example like me just a bit more than the pope, and the penalty would make them pay their tax to him instead. The numbers are not in my favour for an assassination attempt.

Option two may also have some drawbacks. What happens if my heir is jailed by the aggressor? Would I have to ransom him out (and how does that work)? It might take years for me to set up a plot against him, and it's not cool to have my heir sit in the slammer in the meantime — and even then, I wouldn't know how to free him. Would he be transferred to me when the aggressor suddenly gets whacked of natural causes, or do all his prisoners receive the get-out-of-jail-free-card?

Have I overlooked any options? It would be nice to simply demand the aggressor to denounce his claim, or to give military support to my heir somehow — as far as I can see, my best bet here is option one above, even if it leave less profit$ for myself.

How can I avoid this in the future? In hindsight, I guess I should have given my heir more power right off the bat, but I wanted to expand a little bit to give him a complete duchy.

EPILOGUE
It's been a while, but here is the thrilling conclusion to my little conundrum. I pondered over it, and figured out I had been unwise when dividing up my lands. I had reached my limit before my son became of age, and had ended up giving a strong county to the later aggressor. When my son came of age, I gave him a small recently aqcuired land (means no standing army) as I had planned to expand and give him a whiole region later on.

Now, seeing as the land I had given him was useless anyway, I gave him a better one. Still not enough to win the war, but at least he'd still have a title while I plotted revenge.

Coincidentally, my ruler died shortly after, and the heir became a young king with a strong goal to take out the garbage. I knew the count wanted a duchy, so I planted my spy master there and waited a few years, untill he discovered a plot. I actually failed to imprison him, which may not have been such a bad thing, because I could then crush him with military might, which is more satisfying anyway.

Since I had a claim to my original land, I first revoked that title (I realized, that I might have been able to do that as soon as I became king. Comments?) I then revoked his first title, on the grounds of treachery. This meant I went over my demesne limit, but instead of making the same mistake of handing out lands to third parties, I've decided to hang on to it untill my heir becomes of age (ten years or so). Lastly, I upped the crown authority to medium, which means none of my vassals my wage war on eachother.

Slightly off-topic, but interesting post scriptum
A curious thing happened. I wanted the count to rebel against me, so I could have his head cut off, and tried numerous things, such as making him a court jester and bad mouthing him. Finally, I raised all his units (only a couple of hundred) and sent them off to the holy land, figuring this would not please him. Coincidentally, a prolonged crusade had been going on, and the city it was all about, was unguarded. I moved my forces in. I hadn't nearly enough men to besiege anything, untill a rather large English force moved in shortly after. Since I was there first, I became the siege leader. With such a large army of people I didn't care about, I kept assaulting the walls. Although all my men died, the city was taken in my honour. I didn't bother sending more men, but the sultan never retook the city, meaning I slowly racked up the war score. When he eventually sued for peace, I therefore "won" the crusade with absolutely no effort, and the pope declared months of celebrations in my name!