Any war involving the title as a justification allows for a weak claim to be pressed. I'll admit I'm not sure exactly what that covers, but if you can somehow get one of the target titles you want to go into rebellion you'll probably be able to press the related claim (possibly against the rebellion vassal rather than their liege.) I'm pretty sure Independence wars count, as do (Insert Claimant Here) For Ruler wars. I don't think rebellions against Tyranny or force law changes/lower crown authority would count, unless maybe it was against the tyranny of trying to revoke a title?

..anyway. Send your dude to sow dissent between the kid and his most powerful vassal, and if you're lucky it'll tip the balance that causes a rebellion/murder plot/something else useful against him. The only thing I know you can do that will have guaranteed (not guaranteed useful, but does *something*) results is repeatedly murdering the guy and his successors until the inheritance roulette puts a child or woman on the throne (alternately, murder his successors until the next heir is a child/woman, then try to kill him.) Send your Spymaster over there too. Helps out with the murder plots and might trigger events that help wreck the target's reputation, making it easier to get people in on your plots.