i want to apply for permit with my 2 kids to join my husband who's doing his PhD in China.

Araba, you can apply for family S type. The required documents can be as follows:
1.The passport and copy of its data page
2.The completed application form attached with a recently-taken passport-style color photo
3.An invitation letter from your husband
4.Passport of your husband and his residence permit in PRC (copies)
5.Your marriage certificate and the kid’s birth certificates