I'm not saying they couldn't do it - unlike South Korea, China doesn't care too much for the welfare of the population beyond the basic subsistence level, and has enough resources to avoid outright starvation in NK if they took over. I am saying there's no reason for them to take over, because they now already have all the benefits of control with none of responsibilities of it. It would be literally stupid for them to do it unless they face the imminent danger of either losing control (thus Korean War) or having a hot and possible nuclear war on their borders (thus agreeing to put the leash on Kim, for a suitable price of course).

China has historically tolerated NK because it doesn't want to share a border with a US ally. China intervened in the Korean War precisely because the UN forces came too far north, in spite of China warning them to stay clear.

So this is why maybe the sudden change of heart. All fun and games watching Uncle Sam get poked in the eye until it appears he is going to have troops on your border if it keeps up.

At this point it's in China's actual interest to resolve the situation. They'd probably like to resolve the situation with with the stick poker intact but that doesn't appear to be acceptable anymore. I'm guessing they are hoping Kim cools it down just enough and they can wait it out until the political climate in the US changes and then business as usual.