Silly reader, China controls Kim Jong Un, they aren't going to allow him to deploy a nuclear weapon under any circumstances, they know what the response would be. 
They have far too much to lose in the event of war. What they want is to maintain the North Korean buffer between China and 30K US troops. its the only reason NK still exists.
The real question here is whether Trump will take preemptive action to remove the NK threat. Kim may have already sealed his own fate with his lastest provocations. 
The US cannot allow a rogue regime to possess WMD.