Even if Nordkorea decides to nuke the US. And manages to drop something on US Soil. USA wouldn't nuke North Korea. They would simply start a large scale attack and take apart the whole country with conventional means. China would probably sit back and do nothing except insist that afterwards they need to play a big role in rebuilding it. Maybe China would invade North Korea before the USA to "help" them in order to prevent too much US Influence.