By doing nothing at the moment China may be just raising stakes in future talks with US.
Look at the trajectories: all of them (starting from NK) that end in continental US go over the Chinese territory. So, when the ability of the fat kid to through N-stuff over the Pacific is confirmed, China may invite US to chat in somewhat way like this:
We can buy the latest and greatest S-400 (or 500) from Russia and deploy them along the border. They can easily put down NK ICBMs even at the active (thrust) phase. Would you like us to do so? What would you give us in return (trade, shut up about islands in thee South sea, etc)? Not agree - OK you will have to clean up Alaska or/and Canada from debris at your own (if you are sure GBI can do their job 100%, which of course no one knows).