If I have multiple children, once I die will all of my holdings be inherited by my sole heir? Or will it be like in CK2 and it will depend on the inheritance laws of the kingdom I am in. For example, it could be Primogeniture in which the oldest child inherits, or Gavelkind in which holdings are distributed among all children, or maybe even Agnatic laws in which only males can inherit.
Depends on the inheritance laws, your will etc. Pretty basic stuff. If its your family and only you have the child codes, then only you can spawn in and if you have it set to your child inheriting everything then you're set.