The reason North Korea exists in the first place is because China didn't want the west at their borders due to the stance on communism.
China likely would deal with them in a more subtle way rather than having to take over the country or risk other countries doing it. My understanding is that to even raise NK standards of living to the levels the rural poor in China have, it would be costly to China. Not doing anything could be costly to. I think China will walk a fine line and do as little as possible until it cannot pussy foot around much more. Hopefully it is taken care of before it gets to that level.