North Korea would collapse very quickly without China's support in fuels and food.
China currently won't accept a unified Korea under South Korean rule (which they view as a US proxy). China also doesn't want a destabilized North Korea on its border. But it is likely that many outside China and the Koreas view China's influence as a lot more than it is.
China had Kim Jong Un half-brother under their protection as a way of providing a credible threat of threatening regime change. That isn't an option anymore.
I'd note this quote from a well-sourced FT article from last year:
Beijing has often talked up its influence over its recalcitrant neighbour in its discussions with Washington, but when Pyongyang claimed to have tested a hydrogen bomb this year, Beijing had no idea it was coming, according to several people with knowledge of the matter.
and this:
It may seem ludicrous to outsiders, but several close observers of North Korea told the FT that, given the choice, Mr Kim would prefer an alliance with America, the far-off superpower, than China, the ancient oppressor and emerging superpower.
https://www.ft.com/content/551bb080-1c3c-11e6-a7bc-ee846770e...
Yes, China has cooled down the relationship recently.