China wouldn't be interested in sending in soldiers into a conflict on the Korean peninsular unless it felt that it was directly threatened. It already had contingency plans for the removal of the top echelons of the North Korean state, presumably because a war would present itself as an opportunity to establish a proper buffer state in Korea without having to try and pull leverage on the Kim regime.
North Korea on the other hand has about 5.7 million people under arms at any given time, which largely means that based on numbers alone it is larger than South Korea's forces. However, North Korea is also not interesting in attacking because of a few things. One is that attacking would not garner it support from anyone and it would therefore be alone in its endeavours. Two, the only thing the Korean People's Army has going for it is size. It's equipment is hideously outdated and its capacity to fight is pretty much nil. It is slowly modernising but has to rely on clandestine methods to acquire new technologies. In the event of a conflict it would lose out due to a loss of air superiority from better aircraft in the US and South Korean air forces. Three, North Korea is planning for a defensive war, because it has limited power projection and it has virtually no way to sustain any offensive operations. Hence why it is trying to make itself look as reckless and as scary as possible, as well as developing nuclear weapons. It's all part of North Korea's deterrent. The military brass know it can't win a war and so it's best defence is deterrent. Four, and finally, war would mean the loss of the gravy train for the privileged few in Pyongyang. And they, like anyone else in positions of power, want to retain that control and that wealth.
You are right in saying that China is not an expansionist state, but that doesn't mean it wants to exert influence over or control territory elsewhere which it believes is important. That's why there's division over who owns what in the South China Sea. It's why China is calling itself a "near-Arctic state" in order to try and claim some legitimacy for when it sticks its nose into the grab for territory in the Arctic. It's why China is investing heavily in the developing world. Because it wants control, and the best way to do that is to try to establish some control over natural resources. As for military pressure, China will continue to act belligerently with its military not because it believes it can succeed but because it wants countries like the US and others to take it seriously as a military power. That's why it shows of its new carriers or new submarines or makes boasts about leaps in defence technologies: because it wants to be given that prestige that comes from being a global power.