Does the petitioner have to be present at the interview?
No, only applicants must appear to be interviewed.

If a derivative applicant is following to join the principal applicant, do they have to be present at the interview?
Only the applicants who are listed on the interview appointment letter issued by the NVC must appear to be interviewed at the scheduled time. Applicants who will follow to join the principal applicant later will be interviewed separately. You should contact the Embassy/Consulate directly to arrange separate interviews.