Why would China invade North Korea? It is doing what China wants.
China is a major ally of Pakistan, whose China-assisted nuclear weapons threaten India. China is an ally of Russia, whose nuclear weapons threaten the U.S. and Europe. China is an ally of Iran, whose China-assisted nuclear weapons development threatens Israel and Saudi Arabia, both U.S. allies. In other words, China assists all the major nuclear-armed countries that oppose the United States. 
Both Russia and China have warned against any U.S. attack on N. Korea.
 Trump (aka Adolph Twitler, aka The Mango Mussolini) can swagger, harrumph and talk like Dirty Harry to the Asian potentates, but it is all theatre.
Ain't nothing going to happen.