I am a realist here. Lets ask ourselves some questions.
First, is North Korea worth damaging one's diplomatic relations with strategic trading partners in North America and Europe?
I do not believe so. Russia may be a big trading partner, but Russia is not everything and everybody.
Second, is China's military as ready and experienced as the South Korean military or the US military? The South Koreans have been in a cold war with North Korea since the 1950s. They know their geography, the choke points, and kill points of the land through and through. Same goes for North Korea.
As for the USA lets see: Korea, Vietnam, Iraq x2, the Balkans, and Afghanistan. No other county has the sheer amount of experience in modern, urban warfare as the United States of America. Also, drones anyone?
What military experience does the Chinese military have besides marching in parades and bullying some Tibetan monks who would sooner set themselves on fire than pick up a gun, much less a bo staff?
Third, can China afford to militarily intervene in a war in the Korean peninsula?
Sure, just like Iran could afford sending expertise and weapons into Iraq. Whether they have the balls to send troops wearing Chinese fatigues is the question. Then again, one can tell a Chinese from a Koreanv very easily, while the difference between an Iranian and Iraqi is not as clear cut. If China did send troops not in uniform, they would be killed and exposed.
Honestly, I don't think China would risk much if the Koreans went at it. If they backed North Korea full on, then they could kiss Europe and North America as a market good bye. Good luck supporting a 8 to 10% growth and a growing middle class by selling to Russia and a bunch of third world nations.