It's in China's own interest to monitor the nuclear situation. There is home grown terrorism in Western China that would have a vested interest in acquiring those weapons, and China is against nuclear proliferation in the region.
VOA News - US Lawmaker Confident China Will Help with North Korea
VOA News - China Urges 'Coolheaded' Response to North Korea
China will not risk harming its own development over NK, I can almost guarantee it. This is especially true if the U.S. plans to get involved. China's main interest right now is making sure its U.S. investment is proceeding smoothly.