The Chinese can't even stand having starving North Korean refugees here (unless they're being used as prostitutes or cheap labor), so I doubt they give a rat's ass about North Korea. There's no money there.
On the issue of North Korea's nukes, this issue serves China. Because China is included in the multilateral talks, it demonstrates to the world that China is going to be, if not already, the first and foremost player in the region. Above the "democracies" of Japan, South Korea, and Taiwan. China holds a lot of sway, and their influence is growing.