What is more vital for China and Russia, being on good side of probably the most powerful country in the world, or defending North Korea? If given the chance, these two countries would side with the US, seeing as how there's not currently a real hate pack against the US (like Iran), and they have more opportunity for economic growth in the US than they ever would have in North Korea.
As for this whole situation, the only thing that concerns me is the capabilities they have now. They are further along than they ever have been before, but the US is likely increasing the missle defense progam by a ton. and if they tried to do anything they would be blown off the face of this planet. As for the people bashing Trump for the comments he made, realize the situation at hand and get over your bias already. What did you want him to do, sit on his hands and ignore it like Obama?
See what I just highlighted.
Why would they defend them if:
1. North Korea strikes first even though the US did nothing?
2. America provides economic benefits to both China and Russia that North Korea could only dream of?
China and Russia are starting to see the writing on the wall, it wouldn't have been a 15-0 vote if they didn't. North Korea's regime is on the ropes. It would honestly not shock me if China start a coup de tat and killed Un while installing a puppet government. I see this happening way before any sort of war (let alone WW3) break out.