How to get visa if residential permit wasn't renewed?

I am a student in a Chinese University. My wife is also studying there. Recently we had a baby and thus my wife had to come back to Bangladesh. My wife's residential permit was till 25th March. She didn't renew the residential permit. Now she wants to come back to study and also wants to bring our child with her. How can we do that? What kind of documents do we need. Can you please help?