We can't know, but there's no reason to suspect they are. Why would China want a North Korea able to nuke Beijing? Why impose sanctions on North Korea if they're following your orders? And when has North Korea ever listened to anyone?
Interestingly enough, I see the emergence of common ground between Washington and Beijing on this issue. Sooner or later, China is going to calculate that Kim Jong-Un is more trouble than he's worth. When that happens, I can see the US and China arranging an unwritten agreement to take out Kim in exchange for the US's acceptance of the legitimacy of the North Korean state.
That would be a bitter pill for the US and its allies to swallow. But it seems preferable to the current course and speed of events, which basically have Kim developing usable ICBMs within a decade or less. To assume that a growing nuclear arsenal will somehow make Kim less of a threat, or endow him with a newfound sense of global stewardship and responsibility, is to place a particularly strange bet.
Hopefully we can encourage or entice China into forcing systematic reform on a post-Kim DPRK. I don't think China will all too keen to foster democracy and open markets, so we shouldn't expect too much. But we can probably inveigh on China that an economically and socially stable DPRK is preferable to an unstable one, and that the global soft power China desperately craves will come when it is seen as having at least a shred of concern for human rights. Up to us and China to come to terms on exactly what the bargain would be. But my best guess is that it won't be something we'd currently find conscionable or acceptable.
My point is, from a realist perspective, our bargaining power is only going to incrementally diminish over time. Today we have the strongest hand of cards we are ever going to be delt, and it is a really shitty hand. Tomorrow our hand is going to be even shittier, and China's stronger. You dont need to stretch your imagination too far to see that we have no real way to win this game. Previous presidents have tried checking. Trump has tried bluffing. Sooner or later, someone at the table is going to call.