I live in Canada and visit the US often with my children. Each time, I complete a "Permission to Travel" document. I just returned from a two week trip, for which I was asked for the documents when crossing the boarder.
The document is available online. It will include details of the child and the persons they are allowed to travel with or stay with abroad. Names, addresses, date of birth and passport information is included. The details of the parents/guardians are included. Details of destinations, including addresses, contact info and length of stay are included. This letter can either be notarized or signed by a witness over 18.