Unfortunately self-representation is only allowed if the party is representing their own self; they cannot represent a child or an adult child as that requires a license to practice law in that state.
Here is the rule:
Rule 46. Attorneys
(a) Admission to the Bar.
(1) Eligibility. An attorney is eligible for admission to the bar of a court of appeals if that attorney is of good moral and professional character and is admitted to practice before the Supreme Court of the United States, the highest court of a state, another United States court of appeals, or a United States district court (including the district courts for Guam, the Northern Mariana Islands, and the Virgin Islands).
(2) Application. An applicant must file an application for admission, on a form approved by the court that contains the applicant's personal statement showing eligibility for membership. The applicant must subscribe to the following oath or affirmation:
“I, ________________________, do solemnly swear [or affirm] that I will conduct myself as an attorney and counselor of this court, uprightly and according to law; and that I will support the Constitution of the United States.”
(3) Admission Procedures. On written or oral motion of a member of the court's bar, the court will act on the application. An applicant may be admitted by oral motion in open court. But, unless the court orders otherwise, an applicant need not appear before the court to be admitted. Upon admission, an applicant must pay the clerk the fee prescribed by local rule or court order.