To just throw out the other Kims because of parental favoritism is just plain Kim Jong Unacceptable!
Straighten up your behaviour NK! You want to be a monarchy disguised as a communist dictatorship? Then you need to take some not quite preferred kings every now and then.
Just because most monarchies have a law clearly delineating the line of succession (whether it's Salic law, Semi-Salic law, agnatic primogeniture, male-preference primogeniture, absolute primogeniture, or whatever) doesn't mean they have to. North Korea is just a monarchy which allows the monarch to designate his successor. (Francoist Spain was technically like that, too--Juan Carlos's father had a stronger claim to the throne of Spain than Juan Carlos did--but it didn't last after Franco died.)