If a child (under the age of 18) is traveling with only one parent or someone who is not a parent or legal guardian, it’s important to know what paperwork should the adult have to indicate permission or legal authority to have that child in their care.
Due to the increasing incidents of child abductions in disputed custody cases and as possible victims of child pornography, Customs and Border Protection (CBP) strongly recommends that unless the child is accompanied by both parents, the adult have a note from the child's other parent (or, in the case of a child traveling with grandparents, uncles or aunts, sisters or brothers, friends, or in groups*, a note signed by both parents) stating "I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter/group. He/She/They has/have my permission to do so."
CBP also suggests that this note be notarized.
While CBP may not ask to see this documentation, if they do ask, and you do not have it, 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, while the U.S. does not require this documentation, many other countries do; failure to produce notarized permission letters and/or birth certificates could result in travelers being refused entry (Canada has very strict requirements in this regard).