Overkill however if you want to cover all the bases:
Consent to Travel
In the United States, children do not typically need to carry a written consent to travel with an adult. If the courts are involved in the child’s life, his ability to travel might be affected. Both parents should sign a notarized consent to travel. You can download a form online or create your own. Specify the child’s full name and address, the names and addresses of both parents, the names and addresses of the adults traveling with the child, the authorized destination countries and the dates of the trip.
Medical Consent
A medical consent form gives the accompanying adult the right to authorize medical treatment for your child while he is away. Some people combine a medical consent form with a general consent to travel form.
A copy of their medical insurance card will save you from having to pay or provide your own payment guarantee (except for a co-pay) in case of need of medical treatment.
Temporary Power of Attorney
If the trip will include activities such as parasailing or dog sledding that require parental consent, consider signing a temporary power of attorney. This is a legal document that gives the child’s adult travel companion the right to make virtually all parenting decisions for the specified period of time. A temporary power of attorney must be notarized.
CAVEAT
This list is written from the American perspective where society has moved toward less trust, reliance on each other and handshake agreements. Everything is becoming more legalistic and subject to litigation. In many countries this list would be viewed as absurd.