The best is to call your local ER and see what they would want included on such a letter. We leave one at my mom's and my DMIL whenever we leave the kids. The first time we went on a trip without our kids, our son (10 at the time) crashed his little dirt bike and broke his arm. My mom took him and the letter to the ER, and the receptionist laughed and said "These parents sure know their kids!" We had hoped no one would need the letter.......The receptionist said the letter saved so much time and work.
You should definitely have a letter stating medical consent, signed and dated by the parents, (don't forget to include any special medical conditions on the letter) and a copy of their insurance. Chance are you will not even need it, but why take the chance!
Have a great tirp!