how to gain a visa to visit my child father in PRC?

Sasha, you can ask your husband to issue an invitation letter to you. In this way, you can apply for a S type for you and your child to visit the child father in PRC.
The first step is to prepare all the required documents including the passport and copy of its data page, the completed application form attached with a passport-style photo, an invitation letter from the child father, and copies of his residence permit and passport, your marriage certificate and your son’s birth certificate. Next, submit your application to the embassy of PRC to apply for it.