Unless I'm wrong, the U.S. has the ability to watch, from satellite, every one of the large missiles that North Korea puts on a launch pad, and it takes probably 1 to 2 hours for one of those things to warm-up and get ready to launch.
Incredibly easy, therefore, for us to simply send in a small guided missile (10 if necessary) and kill their missile DOA on its own launch pad. One really huge explosion. Missile gone. Launch pad gone.
We don't need to wait for it to be airborne to try to destroy it.
China would do nothing. They live off of our consumer purchases, and hold billions in U.S. Treasuries that they are collecting interest on. They are rapidly becoming middle class on a national basis, and aren't going to bleed green blood just to help a mad dog like North Korea. China has been pretty consistent in only doing what is in THEIR best interest, unlike the U.S.