China's actions if hostilities flared with N. Korea are hardly irrelevant but they are not the main problem, nor are N. Korea's nukes. N. Korea has 12,000 artillery pieces, many of which can range Seoul, a place with around 25 million people living in it at population densities that make it nigh impossible for a shot not to kill at least several people. Even the most barely trained crew can line up a shot that will hit in that area and shove multiple shots out per minute. Several thousand artillery pieces firing are essentially dropping a nuke per volley, except nukes aren't as deadly in terms of raw explosive yield as a parallel artillery strike.

It isn't fear of China or a nuclear war that keeps us, or S. Korea on its own, from pounding N. Korea flat. S. Korea could kick the shit out N. Korea on its own. It is that even if everything goes perfectly with planning and implementing a first strike without them getting a whiff of it, you still need to kill 99% of their guns practically simultaneously just to have a hope of keeping your civilian death toll to 9/11 figures. Just to give an idea here, a lone artillery battery, just 6 guns, could do horrible things to any city in mere minutes, they've got 2000 times that many.