China is not the US. It has its own regional problems. North Korea is a much bigger problem for China than Mexico is for the US.
I think China has North Korea fairly well in hand. I also think North Korea's sudden spate of missile launching had more to do with Trump's plans to threaten Chinese exports than anything else. Once those threats stopped the missiles stopped. If Trump decides to resurrect his threats and threaten to take action against China's trade violations expect there to be more missile launches from North Korea - which China will 'handle' as long as the US shuts up about trade.