China would not be happy, to put it mildly, with an American/South-Korean attack on North-Korea, due to the perceived insult it implies to them and obviously due to the deployment of American troops so close to them. But it's absurd to suppose that they would go to war again on the North-Korean side. They have no interest in isolating themselves diplomatically and, as an inevitable consequence, economically, to such an extent. For the same reason, even if you suppose China would instead take on North-Korea itself, it would not be nearly crazy enough to invade South-Korea in the process. It's one thing to talk tough and demand to be treated with the respect due to a superpower, but quite another to go about attacking neighbours and making themselves look like a dangerous irresponsible state on whom you certainly don't want to rely too much for trade.