One more thing

If you parents plan to use AP, they should not stay in China for too long before coming back again. If they stay too long in China the port of entry immigration officer may question their intent.

If your parents need to stay in China for an extended period of time, the best way is to have them wait for GC process in China. You can either start the case here and transfer it to China, or have them go back now, and start the case as immigrating them while they are in China.