If you're traveling as a solo parent with your child, or if the child is traveling with a grandparent or guardian, be aware of documentation an adult should be prepared to produce. At a minimum, the traveling parent should carry a letter of authorization from the absent parent, granting permission for the trip. It's quite likely that, for most countries, you won't be asked to show any travel consent letter -- but better safe than sorry, right?
Which Countries Require Travel Consent Documents?
A good resource is the US Department of State's Bureau of Consular Affairs site section that lists the travel documents required by numerous countries around the world.
But What About Travel Between Canada and the US...
...where cross-border travel is so common that people sometimes change countries for a shopping trip? As of this writing, the Canada Border Services Agency site says:
"Divorced or separated parents should carry copies of the legal custody agreements for the children."
Also: "If you are traveling with minors, you must carry proper identification for each child such as a birth certificate, passport, citizenship card, permanent resident card or Certificate of Indian Status. If you are not the parent or guardian of the children, you should also have written permission from the parent/guardian authorizing the trip. The letter should include addresses and telephone numbers of where the parents or guardian can be reached."
It's always a good idea to check the latest info about document requirements, in case the security climate changes. For entry into Canada, the US State Department site is a good source, or try the Canada Border Services Agency.
Preparing a Letter That Authorizes the Child's Travel
A "permission to travel" letter doesn't need to be a complicated document. Basically, its purpose is to state that one parent gives permission for the child to travel out of the country with the other parent (or with another accompanying adult) on specified dates.
Family Travel Forum has a handy online form you can print out for a Permission to Travel letter. It includes places for a Witness to sign and for a Notary Public's signature or seal.
You may wish to take that extra step and have your documents notarized if your situation is complicated. A "notary public" is a person authorized to legally witness another person signing a document. In the US, this service can commonly be found in banks, real estate offices, local government centers and elsewhere, and fee is minimal ($2 to $5, at time of writing.) In Canada, the fee is much higher ($25 or more) and you'll need to locate a notary public, typically at an individual office.
Keep in Mind, When Crossing Borders:
Requirements for documents can change over time. So do check the entrance policies for the particular country you're visiting. The U.S. State Dept.'s Bureau of Consular Affairs lists entry requirements for many countries. If you're still uncertain, contact the Embassy or Consulate of the country you're visiting and ask what documentation you'll need.
Additionally, the level of caution in an airport may increase even without a formal change in policy. For example, if a high profile case of cross-border child abduction has been in the news, expect that border officials will be extra vigilant.
And finally, if your situation is anything but straightforward (for example, a child has a different last name than the parent he/she is traveling with), be extra careful about documents and paperwork. In a case of differing last names, you'd be well-advised to have a notarized letter of consent from the absent parent, and a copy of the child's birth certificate that specifically lists you as a birth parent. (In Canada, this is called the "Long form" birth certificate.) A mother who's remarried and taken a new name has another layer of responsibility added: she should be prepared to prove she is the person named on the child's birth certificate.
*Canadians traveling to Mexico: note this extra requirement for kids under 18 traveling without a parent.