North Korea and China

China could impose great economic and military pressure on North Korea but is unwilling to do so, fearing a precipitous collapse of the North Korean regime. This collapse could lead to even more insecurity on the peninsula and waves of refugees pouring across the border into China. It also could draw the South Korean military and its U.S. allies into North Korea -- at China's doorstep.

As a result, China opposes calls for stringent international sanctions and military action against the North. China's U.N. Ambassador Wang Guangya put it this way: "I think there has to be some punitive actions, but also I think these actions have to be appropriate." He said the U.N. Security Council must give a "firm, constructive but prudent response." China's foreign ministry spokesman, Liu Jianchao, says military action is "unimaginable."

As one of the five permanent members of the Security Council, China can use its veto to prevent robust Council action against North Korea.

South Korea

South Korea shares China's concerns about a possible collapse of the North Korean government. In addition to the likely surge of refugees, the economic costs of stabilizing and perhaps reuniting with the North would be staggering. The burden that West Germany incurred by unifying with East Germany pales in comparison to the probable cost of reunifying North and South Korea because of the extreme disparity in wealth between the two.

South Korea also opposes military action against the North. A military conflagration would be devastating to South Korea. Sometimes overlooked in the debate about North Korea's nuclear weapons is its large conventional force. Analysts doubt that the North could sustain a prolonged military campaign, but it could do tremendous damage to Seoul -- which is within artillery range of the demilitarized zone, or DMZ -- in a very short time.

In recent years, South Korea has pursued a "sunshine policy" of engagement with the North, promoting trade, tourism and dialogue across the DMZ. South Korean President Roh Moo-hyun says that policy will be reviewed.NPR

I think this is an accurate picture of the event.
China has other reasons to prevent North Korea from collapsing. Ethnic Koreans live near the Chinese border, so if unified by South Korea, an independent movement among ethnic Koreans in China might take place out there in China. And China does not want to see it happen because of the domino movement of independence of the minorities in China.
Besides if my memory is correct, China and North Korea have the treaty of the alliance to the effect that if one nation is invaded, another nation will come over to protect. So that is another reason China is against the article 42. But the treaty does not apply to the case where China militarily intervene because the treaty does not mention the case where China invade North Korea.
My suggestion is let China militarily intervene North Korea backed up with the UN resolution and let China replace KJI regime with Chinese puppet regime. The point is whether the US and South Korea will approve it.