N. Korea doesn't have nukes....yet. Even if they did, their best missile barely cleared Japan before crashing into the sea. It's more likely that IF N. Korea had a nuke (*A* nuke) and decided to use it, they'd use it on Seoul as a "take as many of them with us" type of suicidal strategy. N. Korea will get one shot and only one shot before S. Korea and the US retaliates - and retaliation will take the form of conquering the country.
Use of nukes by N. Korea would DEFINITELY bring China - and maybe even Russia - down on them. In fact, by the time US troops would arrive in the capital of Pyongyang , they'd be just in time to watch China raise its flag over the remains of the main government building, while kicking the bullet-ridden corpses of the Il Jong family out the front door. China doesn't believe in kangaroo court cases. They'd most likely execute everyone who is part of the ruling elite as soon as they captured them.
As for the US, we won't use nukes on N. Korea - not even small tactical ones. It's not warranted and the collateral damage will just be too great. The N. Korean army isn't in all that great of shape, nor is it very large. I also have to wonder just how many would remain loyal to a leader who basically signed a death sentence for their country by actually attacking.
No, N. Korea will go back to its usual passive-aggressive game. They'll back down, as "a favor to the West." The moment N. Korea actually crosses the line, at a minimum they'll be facing the combined forces of the US and S. Korea - both of which are way better trained and equipped (AND FED!) than the N. Korean forces.