John Ashtone above me is right, I think.
China will not want the USA meddling in the region, and will send a delegation to the American Embassy not to get involved. America will then say to China - well, can you do any better?
If Kim Jung Un does not stand down, then in the fraternal interests of the peoples of the region, and our common cultural bond and to restore peace and harmony in the region, the Chinese red army can invade, and the Americans will sit on their hands when it comes to the UN vote.
What would be interesting is what happens next. South Korea and China have no major disagreement, being more competitors than enemies. It would be in China's interest to ask South Korea to consider a much closer trade and political harmonisation in exchange for control over North Korea and sovereign autonomy for the Liberated Korean People's Republic, with the South Korean flag, symbolising the Chinese gift of harmony (and the Chinese no longer wish to eliminate their ancient cultural emblems, wherever they serve the People).
How would America or Japan react to this? Would either be strong enough to put up any more than a token diplomatic protest?