I think that behind the scenes China and South Korea are cooperating on plans for the post-Kim era in North Korea.
China knows the future is with a united Korea with the current South Korean government in control. China wants Korea in their sphere of influence, and they are trying to make that transition as smooth as possible.
While that may be true, China is not going to agree to a unified Korea IMO unless the ROK agrees to boot out all US military personnel. The last thing China wants is US troops on its border. Probably one of the biggest reasons China tolerates NK is that is serves as a buffer to US forces in the ROK