China wants to be a benign Asian power more interested in trade surpluses and a healthy than in proping up an idiot like Kim on its border who is gathering international condemnation by the box full. China doesn't swell with joy at the prospect of having its major cities disappear in a nuclear cloud.
Same with Russia. Russia and China don't want an arms race near their country, and don't like the prospect of a nuclear armed Japan and South Korea, not to mention prepositioned US nuclear forces in the region.