
While CBP may not ask to see this documentation, if they 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 many countries require such documentation, and failure to produce notarized permission letters and/or birth certificates could result in travelers being refused entry. For example, Canada has very strict requirements.
Effective June 1, 2009, children under the age of 16 (15 or younger) will still not need a passport, although if they are not traveling with both parents, they will be required to have a letter of parental consent. Teens between the ages of 16-18, if traveling as part of an adult-supervised school, religious, cultural or athletic goup, will also be exempt from rules requiring a passport, although they will need to have a copy of their birth certificate. If just traveling with friends or family, 16-18 year olds will need a passport - as will all other U.S. and Canadian citizens over the age of 15.