China will not stand by if the DPRK is attacked or if another country is attacked by the DPRK. China is into making money and if the ROK is attacked, well, the ROK was good for USD $90.1B imports from China in 2015, and the ROK exported USD $131B to China that year.

On the other hand, the DPRK imported USD $2.34B from China and exported USD $2.95B to China in 2015. (My source is http://atlas.media.mit.edu.)

Fraternal Marxist bonds be damned, China isn't going to lose a USD $88B market because of some lunatic kid...and that kid only remains in power because China wants the DPRK as a buffer state. Should that kid become too much of an international nuisance or start a war (fergawdsake!), well, there are more tractable people available.