I think neither Russia nor China care as much about keeping Kim in power as they do about keeping a modern, productive, westernized South Korea from expanding into the north to their borders. They like having a buffer zone more than they like North Korea per se.
China has no fear of a united Korea. In fact, it is in their interest that this happen since it will increase the business they do with both.
China’s overriding concern here is starving refugees which is why the troops were moved into position to block them.