Just bomb kim. The south koreans will have new lebensraum in the north.
China and Russia couldnt do shit in a real fight. They are only a bit better at posing then the north koreans.
Neither of them would risk their butts for the kim family. Especially the chinese because they know that they will be the future superpower - there is 0 reason for them to risk a war.
Additionally NK is pretty much getting welfare from china.

Russia just wants her sphere of influence untouched. Kim doesnt matter at all for her.
Game of thrones in real life.