if NK ever managed to actually detonate a nuclear bomb even China wouldn't hestitate to march in and take over. I think they'd be glad of the excuse, really.
FYI, North Korea has made three underground nuclear detonations in 2006, 2009 and 2013. Very few doubt that they now got a few nukes in the kiloton range - basically 1940s tech - and the means to deliver them to Seoul - a mere 35 miles away from the NK border. China doesn't care. They got a loyal ally, they could crush him at any moment and it'd only create hostility between Koreans and Chinese. And the country is not worth the trouble. I guess if China ever went on the offensive they'd gobble up NK - and probably SK too - but only if they're on the warpath anyway.