At first, China was reluctant to discipline North Korea, its ally that frequently challenges the US its rival. China probably wouldn’t have minded if North Korea got one or 2 nukes.
But since Kim Jong UN’s country developed a hydrogen bomb, the senior brother of the atomic nuclear bomb, China itself has gotten very concerned: that North Korea may even grow powerful enough to boldly challenge it.
This is good news to Donald Trump and the US, because it is only China that really has any influence on Kim Jong Un and his government. North Korea may be forced to freeze its nuclear programme, but China will still keep the Kim government as a bargaining chip against the US-as long as Kim doesn’t turn around to openly challenge China.