If a minor under the age of 16 applying for a US passport does not have a parent present, then the U.S. Department of State allows a parent to complete a letter of consent (the DS-3053 form) instead. This form must be notarized. Need a minor travel consent form, or to notarize one you've already filled out? Regardless of how much progress you've already made, Notarize can help you out:
To complete this document with Notarize, you must successfully identify yourself, fill out the document, and sign it before one of our licensed remote electronic notary publics located in Virginia. They will witness your signature and notarize your document.