If you have migrated and show them your PR visa, they will not make any difficult to your wife about sencond or third kids.
I apply migration to Australia becasue my wife pregnant a second kids and my visa approved after my son one years old. I am living at one of Shanghai, Guangzhouand Beijing and I tell them I am applying to migrate to OZ and they never make any trobule to me.
You are the OZ PR, your wife have the special right for the second kids or third kids. that is one policy of the 计划生育