All you have to do is send a paper with her giving them permission to authorize treatment, with their names and your daughter's, your signature, and a legible photocopy of the insurance card. You've probably signed forms like that for school, camp, sports etc. a dozen times. I've had to make forms like this for parents of the kids in my youth ministry when we went on away trips. This is all you need:
"In the event of an emergency or non-emergency situation requiring medical treatment, I, (Your name), hereby grant permission to (grandparents names) to authorize any and all medical and/or dental attention to be administered to my child (her name) until such time as I can be contacted. This permission includes but is not limited to the administration of first aid, the use of an ambulance, and the administration of anesthesia and/or surgery, under the recommendation of qualified medical personnel."
Sign and date, keep a copy for yourself and give them the original w/ink signature, attach a (front & back) copy of your insurance card, and that's it. It is also be a good idea to list any allergies or medical conditions your daughter has. It wouldn't hurt for it to be notarized but it's not absolutely necessary.
Source(s):
Countless overnight trips with teenagers as a youth minister