They probably would... but the key is how much, and how overtly?
The thing is, anything short of overt war by China against the US/South Korea/Japan in that context would do absolutely nothing to prevent North Korea from collapsing under US/South Korean attack. This isn't a "supply them weapons and they'll fight a proxy war" scenario. The North Korean regime is not capable of non-state-actor-type tactics, and the US and South Korea are each individually far more militarily capable than, say, the Syrian government.