Reposting the top comment of this thread which is really good: https://www.reddit.com/r/communism101/comments/ahvu9h/is_north_korea_a_hereditary_dictatorship/

The fact is that Kim Jong-Un, and his father, and his grandfather, do not and did not hold the highest positions of power in the government. The de facto head of state in the D.P.R.K is currently Kim Young-Nam, who has held this position since 1998. The current position of Kim Jong-Un is as you said, as he previously held the position of Chairman of the National Defense Commission, which was altered in 2016 to better reflect the situation at hand with better organisation. Kim Il-Sung held the positions of Premier and President of the Presidium for a long time, though he did retire from the latter in 1983, being succeeded by Yang Hyong-Sop; this is unsurprising, as Kim Il-Sung was a incredibly competent and well-respected leader, and especially so among his own people. The fact is, influential positions have become more decentralised with each generation, as the power of the state becomes more distributed among the proletariat and in accordance with society, in a rather natural way that may, hopefully, lead to a complete withering way of the state in future, as other aspects, too, decline, assuming a defeat of capitalist powers simultaneously with the existence of the D.P.R.K.

Allegedly, Kim Il-Sung did suggest Kim Jong-Il succeed him, because Jong-Il had been instilled with a revolutionary ideology for his entire life. The rest of the party and the people are said to have agreed with this - through their usual discussions, one would assume - and so, Kim Jong-Il succeeded his father in some positions, whilst others were removed, split up and redistributed, or reassigned. That is why there is no longer a President of the D.P.R.K: The position was no longer necessary, and equivalent powers were to be divided among multiple roles.

On your point of their democracy: The decision of whom is elected is not done so with the ballot. The voting is done through open discussions prior to the ballot, comparable to the Soviet design, which could also be seen in Libya to some degree. After a common consensus is met through these periods of discussion, the vote is cast as a confirmation of this selection, hence the lack of negative votes for candidates; democratic centralism requires unity, after all, and so does a socialist state. You can find many posts here that touch on the matter. I recommend this for some more information.

So, in short: The D.P.R.K is not a hereditary dictatorship, with power being decentralised through successive generations, and the election of officials are made through common consensus as decided by mass discussions.

Hopefully that helps.