Not to go too conspiracy, but why don't Russia and China get together and just kill Kim Jong-un and his family? Then install their own leader.
They need that buffer zone for sure but right now the problem is Kim Jong-un is creating too much noise and commotion on the world stage. If he keeps going, eventually USA will get involved and there's no way that ends up good for China or Russia.
On top of that, I am sure that even if Russia and China use some foul play(go against the rules of war), USA and the international community would look the other way and let it slide in return for a stable non-nuclear launch every month leader.