other requirements for children

Good day, my husband and I and also two of my children (we have four children) already had previous china visa. We will visit china in December, are there any other requirements needed for my other two children who are going to apply for china visa for the first time, besides their student id and sponsorship letter? Also, can we just make one sponsorship letter for the two of them? Do we need to have it notarized? Thank you very much.