Mexican law requires any non-Mexican citizen under the age of 18 to carry notarized written permission from a parent or guardian not traveling with the child to or from Mexico. This includes children traveling with only one parent. This permission must include the name of the parent, the name of the child, the name of anyone traveling with the child, and the notarized signature(s) of the absent parent(s). The US State Department recommends the permission include travel dates, destinations and a brief summary of the circumstances surrounding the travel. The child must be carrying the original letter – not a facsimile or scanned copy – as well as proof of the parent/child relationship (usually a birth certificate or court document) – and an original custody decree, if applicable. Travelers should contact the Mexican Embassy or closest Mexican Consulate for current information.
A similar letter allowing the person bringing the child into Mexico to make medical decisions for the child in the event of a medical emergency is also a good idea.