I know one of the shrines can change a subordinate into a slave. Sorry I can't remember exactly which one, but it sould notify you that you can do it. As for your children... don't know why you want that, but it's your playthrough. If I rememberright, you should be able to give them slave training that lasts a week in game. If that still gives them the subordinate title, than the shrine I mentoned before should still work in theory.
