If it came to war, China would stay out of it until near the end, then do their best to take over as much of the country as they can before America does, and finally walk up to the treaty table on the victor's side insisting on receiving the same share of the spoils as America. The only way they would aid North Korea would be if they truly believed America was planning to use NoKo as a base to attack them, and while they will certainly put that line out for the Chinese public to swallow, the CCP themselves realize America has precious little stake in that (in fact, America has been working hard to keep China propped up for some time, because despite all their bluster they are not dangerous to us but the forces that would be unleashed by their collapse are a different story), and they would have little to gain from fighting us and a great deal to gain from giving us a minimum-effort kind of "support." They're no fools.