Chinese citizen living in US on work visa. What are requirements to travel to China for 3 mo visit?

Our neighbors are Chinese citizens living in the US on a work Visa. The mother has a work Visa and she has 2 children living here with her going to school. THey are traveling back to China for a 2 month visit and we would like to know what forms/Visas, etc. are required for their travel. THank you.