That's an interesting suggestion Ellipsis but isn't the opposite problem about as plausible? Why would China invade its effective ally? The last time the US invaded North Korea the Chinese joined in on NK's side (oh, and Russia helped) and that led to a rather bloody stalement that we were rather lucky didn't turn into a more serious war, and while US-China relations are better these days it's obvious that there's still a fundamental disagreement about how to deal with North Korea between the two. At any rate, I think you're doing Trump a little too much credit. So far all of his "fire and fury" threats have been empty, and treated as such, and look like they aren't measured and considered.
Any strike on North Korea designed to wipe out the entire country, particularly a nuclear strike, would certainly have effects on South Korea and China, either through retaliation or just because that's what nuclear radiation does.