I think a letter of consent from a parent would be of great benefit...and maybe required. I found this article which explains:
When to Carry Consent
A grandparent can never go wrong carrying proof of parental consent when traveling with grandchildren, but on many trips, she'll never need to supply it. Consent forms aren't necessary on domestic road trips unless a grandparent runs into trouble with the law. On a domestic flight, an underage child won't need to present identification of any kind to the Transportation Security Administration, though it's always smart to check the airline's ID policies for children. If a child and grandparent are leaving the country, however, carrying consent is wise. U.S. Customs and Border Protection "strongly recommends" that any child traveling without both custodial parents have proof of their consent. Bottom line: no matter where they're headed, take a few minutes putting consent forms together, just in case.
