There are some really big problems in the way that grandkids are handled in this game.
For one thing it's really easy to lose track of your dynasty more then one generation down the ladder. Giving your eldest son a county pretty much guarantees that his heir is going to marry a nobody and you'll have no say in it. But even if you don't give him land the grandkids won't be able to marry well because they will count as unimportant nobodies. Even if they are heir to an empire they will still count as nobodies because they aren't your children and only titles held by you or your parents seem to give status. Cue "desires a better alliance" then the HRE or ERE.
Also the AI doesn't understand grandkids. It's really easy to swindle a dynasty that had a long lived king. When gramps kicks the bucket his son get's the throne but his grandson get's a claim. This let's you invite the grandson to your court and marry him to one of your daughters matrilineally. After all he's just a courtier and has to marry who you say. Just hope that the grandkids don't marry some random debutante before gramps croaks. Be vigilent enough and you can fill your council with more princes then Aristotle's academy!
These problems are so disappointing because overall the dynastic politics system of the game seems to work really well otherwise. I quite enjoy have the chance to set my chillun's up with that perfect mate. I just wish that it worked without me having to resort to exploitative workarounds after the first generation or two.
I'd like to see some sort of "head of the family" mechanic that would fix these sort of problems. A character should trace their lineage back to their grandparents (or great-grandparents if still alive.) If one of the family patriarchs (or matriarchs) holds a higher title then them that character should be the head of the family and be allowed to arrange them marriages. After a certain age they might run off and marry on their own to keep the player from forcing celibacy on all the second sons but the AI will still judge a character's marriage value by the head of the family not the father/mother. This way more sensible marriages become possible and you don't lose the ability to play dynastic matchmaking just because you lived to be a grandpa.