I think I've made plenty of valid contributions to plenty of threads, I have as much right as you to post so don't tell me where I can or can not post.
It is your fault that your grand children have the wrong culture penalty, you are ultimately resposible for all of your hiers actions, you want more control over them, then don't let them out of your court. You made the choice to send that child off to get claims, not the AI, you did that.
And how exactly should the culture penalty only work in some situations and not others? Just because you gave these people land? If you did it right, they would never rebel, there's plenty of things you can do or not do that would stop rebellions.
Culture penalty is what? -20 at the most, couldn't have been that alone, you did something else wrong, my guess, not de jure liege tipped them over.