A parent permission letter is a document showing that a child's parents consent to the child leaving the country with another adult. U.S. Customs and Border Protection recommend one for all children traveling without both parents.
A parent permission letter needs to contain the names of the adults or group that the child is traveling with, the time period the child is traveling and the countries or general area the child is visiting. Ideally, the letter should also include identifying information such as the child's age. Children who are traveling also need to keep their birth certificates with them. For extra safety, permission letters should be signed by both parents and notarized.