If a minor is traveling with only one parent or someone who is not a parent or legal guardian, U.S. Customs and Border Protection (CBP) requires that the adult traveling with the child carry a notarized letter from the child's other parent (or, in the case of a child traveling with grandparents, uncles or aunts, sisters or brothers, or friends, a notarized letter signed by both parents) stating "I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter. He/She/They has my permission to do so."
While CBP may not ask to see this documentation, if they ask, you need to be prepared. If you do not have the proper documentation, you may be detained until the circumstances of the child traveling without both parents can be fully assessed. If there is no second parent with legal claims to the child (deceased, sole custody, etc.), any other relevant paperwork, such as a court decision, birth certificate naming only one parent, death certificate, etc., would be useful.
Adults traveling with children should also be aware that some airlines and many countries require such documentation, and failure to produce notarized permission letters and/or birth certificates could result in travelers being refused boarding or entry.