I know China has made it clear that if the U.S. strikes first it will side with NK but I don't think they will get involved. It wouldn't be wise for them economically and militarily.
If U.S strikes first NK nukes will be taken down. NK threat to SK will be fought with convention weapons/ missiles and the death toll can be in the tens of thousands but not in the millions.
On the other hand if the U.S kicks the can for another administration to handle NK; then the death toll in the future will be on american soil instead of Seoul or abroad.