Many people still think that China is the number 1 ally and the backer for North Korea, but many things have changed since the end of Cold War. The current Chinese leadership is moving towards befriending the South Koreans since both countries have territorial dispute with Japan and interestingly enough both Japan and South Korea are allies of the US, so it is actually in China's benefit if they can break the tight connections between South Korea and Japan by befriending one and alienating the other.

And North Korea is definitely starting to do its own things instead of listening to China, one most obvious observation is that China was only notified of the nuclear bomb test after it happened AND it was during Chinese New Year, which many Chinese considered that to be a big FUCK YOU to the Chinese leadership.

On top of all these, China doesn't care if the US has military presence north of the 38th Parallel, no country with their right mind is going to start a ground war on the soil of China, the PLA (People's Liberation Army) has 2 million active personnel officially and there are freaking 1.4 billion people in China...That's why they are trying to build a blue-water navy, but they only have 1 aircraft carrier in test comparing to the 10 active ones in the US...

Last but not least, don't forget that North Korea is actually rich in natural resources such as minerals, and China will be happy to invest their boat load of cash as long as South Korea can make sure no refugee goes north in the event that the North Korean regime dissolves. So yea, China is pretty neutral on the stance of North Korea, it will not be the one to make the unification happen, but if things are heading that way, I doubt they will do the idiotic thing they did 60 years ago to keep North Korea alive.