I'm fairly new to this game and just started a Normandie-->England campaign working out quite well. 50 years into the game, most duchies are owned by dynasty members, got Wales and Flandres and I am about to inherit the massive Duchy of Aquitaine with the son of my eldest daughter (which is all of the de jure kingdom) and the Kingdom of Lotharinga with the Son of my eldest Son (which is quite massive too), and I just killed maybe 8 children aged under 10 for that up to this point...
They are both counts somewhere in England as I made this mistake before and didn't land a family member getting Poitou, because I assumed he would automatically join my Empire as a close member of my Dynasty, which is apparently wrong. But as long as characters are landed, your vassals and of a lower rank, they stay your vassals, right? So in this case, all of Aquitaine will go to England?
What about Lotharinga? Will my grandchield be an independent king first but as he is the heir of my heir it will be merged into England later?
Another thing is my growing dynasty. William the Conqueror had 7 children, resulting in about 20 grand-children, and now the 3rd generation is about to grow up adding another dozens of little kids, and I'm at the point to loose the overview after only 50 years and 3 generations. Is it, ahm, common to really manage all this, or should I just marry all but like 10-15 people into other houses to not mess things up completely? And is there a way to open up everything in the dynasty tree with one click rather than clicking on a + 100 times?