You know, I don't even know if there would be a like for like retaliation if Kim glassed Seoul. I mean, aside from Pyongyang (which would probably be improved by a nuclear warhead) there's not a lot worth nuking in North Korea. Their space launch site maybe, but that's probably better off being hit with non-nuclear weaponry than with. Otherwise it's all countryside and minor habitations, nothing really worth expending a nuke over.
Would Washington really nuke the DPRK in retaliation? Seoul must think so or it'd have nuclear weapons of its own, but I'm not so sure that it would make military sense.
Tactical nukes on concentrations of North Korean forces I could see, but then they'd just adapt by spreading them out.
You could nuke North Korean airfields but there's not exactly much of a North Korean airforce anyway and it would just deny the Allied Forces the use of the airfields when they inevitably push the North Koreans back past Pyongyang.
Meanwhile the PRC is hardly going to let a full scale nuclear exchange happen on its back doorstep, chances are that before the mushroom cloud has even stopped rising above Seoul that Beijing will be on the phone to Washington to offer to assist them to remove Kim Jong-un and not to fight against the Allied forces push North into Korea so long as Washington doesn't retaliate with nuclear weapons and that some sort of agreement is made as to mitigate the inevitable economic catastrophe that is about to be unleashed upon the Far East and from there, the world.