As of Feb. 1, 2008, both parents must sign the passport application form DS-11 (needed for replacements too, use link below to get this form) for all children under the age of 16. While this aims to prevent abductions of children by a parent, it could be a hassle if only one parent is traveling with a child whose passport is lost or stolen. If the non-applying parent is unable to sign the DS-11 in person at the embassy, he or she MUST submit a notarized statement of consent authorizing the parent accompanying the child to make the passport renewal application. The safest thing is to prepare for this before you leave. Download the ""Statement of Consent: Issuance of a Passport to a Minors under 16"" form and have the non-traveling parent sign and notarize it. Take the form, and a good copy of the child’s birth certificate, with you for emergencies.
For more information, visit the Travel State website