You can't grant titles to your direct heir, but I just realized you can grant them to your heir's heir.
My character, the King of Ireland, is old and has only daughters. Partition is coming. Because women can't be granted titles in the Irish culture/Faith context, I am unable to hand out specific titles ahead of my death to the younger daughters to ensure an easier succession.
But my eldest daughter has a son, who will be her heir, and I saw I could grant him titles to keep them in the family line. Congratulations, young Matthew; for your 15th birthday I am giving you two Duchies.
He loves me (+100 Opinion); I hope there's not a meaningful risk of him hating his mother and joining factions against her.