Why does Rome 2 need to use claims??
Ultimately, if your kid wasn't up to the task, he'd be ousted by someone who felt themselves stronger and that was pretty much that. So you can only rule the faction you are a member of or declare independence; you won't be able to "press a claim". Si vis pace para belum: In the Roman world, might was pretty much all that mattered, and if you wanted a throne, you could simply try and take it, provided you had the skill and backing. Clearly the son of a king would have a lot of support, but if they didn't have at least enough personal skill to retain support of the military alongside the rest of the aristocracy, they would simply be ousted in a coup or civil war.
And there's no pope to tell you off for attacking fellow Christians. This is Roman vs tribe or Sparta vs Athens without some central laissez-faire authority (which the pope was).
Feudalism doesn't exist ergo claims don't exist. Just conquest, internal and external in a world where might made right.