Children under the age of 14 are required to appear in person at the embassy/consulate for the interview along with both parents. In case one parent is unable to attend the visa interview, the absent parent should provide a letter to the other parent (who is going to attend the interview) that states that they have no objection to the child’s application for a U.S. visa.
The consulate does not permit interested parties, such as friends, relatives, or business contacts, to intervene on behalf of a nonimmigrant visa applicant during an interview. If any such party wishes to present facts to the embassy concerning a particular visa case, they are encouraged to supply this information directly to the applicant for them to bring in person.
US Visa Interview Guide
US Visa Interview Guide - Complete guide for the U.S. visa interview procedure and helpful tips