I think China is wobbly on exactly what to do. On the one hand, NK makes a good ally to hold off capitalism on their border. OTOH, they cannot control NK as much as they would like. Now that NK has nukes, NK no longer needs China (as much) as a military ally. I bet China is very nervous about that.
The question is: What can China do about it? I say "very little". But Chinese politics are multi-layered and you never know which layer is correct and, if it is, will it continue to be correct?
I think China needs NK as a military buffer zone more than an ally to hold off capitalism. China itself is opening all border for capitalism and foreign investments. Why need a trouble maker like NK? I think NK's value to China is minimal.