If North Korea dares to think about actually doing anything wrong (launch nuke, attack the south), China will premptively attack and seize their capital. It's written and was planned years ago.
You have to know that North Korea hasn't any real troops to defend against the Chinese and that the chinese troops in Jilin province (precisely Yanbian Prefecture) and Shengyang province are trained to reach the North Korean capital (Pyongyang) in less than 2 hours. Don't underestimate the number of chinese spies and special forces already there, the chinese korean minority is huge.
North Korea is a buffer state between the US allies and China. One united Korea allied with USA would totally contain China, such a country would control the vast minerals reserves of North Korea and have one top class military. China won't let that happen. United Korea has also territorial revendications on chinese soil and a lot of border disputes. Some south korean parties (including Park's party) wish to control the full Mandchouria, so ...
More importantly, China will never let North Korean refugees swarm the borders and will prefer to take full control of the country, even if it means facing a potential long insurgency in a fanatical country. North Korea pissed off the chinese too. The chinese governement won't feel any remorse.
Last thing, one member of the current chinese Politburo Standing Commitee studied in North Korea. He is named Zhang Dejiang (current Vice-Premier in charge of energy, telecommunications, and transportation).
The older brother of Kim Jong Un, named Kim Jong-nam, lives in China which makes the coup easier. The North Korean governement isn't communist, but nationalist, militaristic (sangun "military first" policy instead of juche) and xenophobic.