They may join the fight, but they could do it defensively.
Cross the Yalu, move up to the front lines secretly as they did in OTL, and then make a big show of it. Do a few counter attacks, and hold the line. Make it seem like a million soldiers are just waiting to attack, but the peace loving Peoples Republic of China doesn't want a war. Then they could make demands for peace.
If they could get the N. Koreans onside, it would leave the North with a very small country while S. Korea is greatly increased.
China would have its buffer, and N. Korea would basically be a Chinese province. It would also be good pr for the Communist controlled China, and would help any future negotiations with the West. Basically the West would probably think, yes the Communist Chinese are Communist, but they're smart peace loving Communists we can make a deal with.
After that all that the Communists would have to do is win their civil war, avoid the cultural revolution and they would be on easy street.