From the US side website, questions, search for “children”
http://www.cbp.gov/
Adults traveling in or out of the United States with children under the age of 18 should be aware of the following:
* because of 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, or friends, a note 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.”
* CBP also suggests that this note be notarized.
While CBP may not ask to see this documentation, if we 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, and 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.)