China could shut down North Korea in a minute.
The do not want to. North Korea is a little yapping dog that can keep South Korea, Japan and the United States distracted all the time, while China pursues a dream of turning the South China Sea into a Chinese lake, and they develop their blue-water navy.
China is not, by any stretch of the imagination, an ally of the United States. Nor shall they ever be, so long as we side with Japan and South Korea.
Not to mention siding with the Philippines.