The DPRK (North Korea for those historically impaired) is China's buffer.. China makes sure to keep all the communities on the Yalu River which borders with the DPRK as poor as possible..
This way, people from the DPRK wouldn't want to "immigrate" to China.. China underestimated how poor the DPRK was because the poorest of the poor in China are far better off
than just about anyone from the DPRK that isn't one of Kim's privileged underlings.... North Koreans still cross the Yalu on a daily basis much like Mexicans crossing the Rio Grande with the
notion that "anything is better than this"....
China is in no way a puppet of the DPRK (as someone mentioned).. China holds all the cards in this situation.. They need the DPRK as a buffer..
China will never compromise it's position on this.... It would be akin to shooting itself in the foot..
Notice how China's rhetoric is harsh at the start but quickly cools and opposes sanctions or any other useless resolution the UN tries to throw at it..
It is not in China's self interest to have yet another westernized country right on it's doorstep...
The DPRK will be the DPRK as long as China exists in it's current state...