If I remember correctly there is an area for the guardian to sign for minors under 16 yo. I believe it had to be in front of a witness. Both times we went to the post office for passports, filled out what we could prior and signed our names in front of the employee who was our witness.
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Re: Passport help
Are you able to do DD by mail? We had to do ours in person for our son because both parents need to be approve?
I believe there was a place for me to sign for him.