Tell China it will immediately lose all access to the US market unless it replaces Kim. China then has a choice of cratering its economy with the attendant internal social unrest or dealing with this idiot whilst having the choice of a responsible replacement so they can maintain their buffer between South Korea and the US.  This is the best choice China will be given. If the US deals with Kim, the replacement will be of their choosing.

China holds billions in US treasury bills so another way will be needed to refinance them to eliminate any undue pressure on the US economy.