I'm no lawyer (someone feel free to step in and correct me), but we have written notes for our children with verbage similar to this:
(date)
I, _______, give permission to __________ to make any/all decisions regarding the health/welfare of my son/daughter, ___________, during the period of ______ to _______. This includes permission to seek medical attention by a licensed physician, and dispense any medications.
(signature)
DISCLAIMER: Again, please note- I am not an attorney, and in no way do I have any idea that the above is legally binding, causes cancer, contains lead, whatever. This is just somehting similar to what we've written for my parents when they take the kids overnight, or when we've watched my nephew overnight, etc. Without something like this, an emergency room will still treat any life threatening illness/injury, but won't administer any pain medication, etc. I recommend contacting an attorney. :)