The first time a traveler is stopped for documentation is typically for a security screening at the airport. Children don't usually need ID at this point, but they do need to have a boarding pass to get through the security gate. The pass proves that the child is a paying passenger with a seat on a flight. The adult traveling with the child is asked to show proof of his own identification with the boarding pass. Although airlines don't typically require minors with a guardian to show identification at this point, check with your airline ahead of time to determine at what age an ID is required of a child, as regulations vary between the airlines.
Whether a child must show identification when boarding the plane depends on which airline you're traveling with. Because the policy for documentation of minors varies among airlines, the adult responsible for the child should contact the airline ahead of time to see if the child is required to show ID at this time. Airlines usually don't require an ID for domestic flights, but if they do, a birth certificate as proof of age is usually sufficient.
In the case of unaccompanied minors, or children traveling without an adult, an ID is required of the adult who drops the child off at the airport and of the adult who picks the child up. Contact the airline ahead of time with the name of the person who's responsible for meeting the child at the destination. The adult dropping the child off should stay with her at all times until she has boarded the flight safely.
Although an ID usually isn't required for children traveling on domestic flights, some form of identification is still a sensible safety precaution to take. If a child becomes separated from his adult companion, an ID can help identify the child and allow for a swifter reunion. If a child is traveling internationally, a passport is always required.
