I don't think that there is a high chance that china would intervene, or else the un nation would just go and give them hell even with alot of people doesn't mean that theyre invincible. China would only intervene because if SK beats NK(I doubt that this is possible) they would inherit North Korea's nuclear weapons, china will be filled with North Korean refugees, and SK will have direct link to China.
I don't think there would be an invasion at all. The best thing for SK to do is build more and more weapons and soon North Korean will run out of resources because China will eventually stop aiding them and NK would fall to their knees, their soldiers would be starving to death, their aircrafts, tanks, ships, subs, etc would be turning to rust. Without aid they would be nothing. And beside they wouldn't dare launch their nukes because after that, they'll know what hell will be like before they die. If it weren't for China, North Korean would be in hell rite now.
They would invade because UN and the US would intervene and And all that Military combined(UN,US,SK, Im pretty sure Japan(They've tasted a nuke and don't want to try it again) and maybe Taiwan(because they've got a bone to pick with communism) would totally overwhelmed NK's nowseemingly small army.