Whatever you take, make sure it is properly notarized. I and another classmate went to Orlando during Spring Break with an adult not related to either of us one year in high school. Our parents both sent notes authorizing the adult to seek medical treatment for us if needed. My classmate got some sort of sun poisoning or something that caused a rash on his back that felt like he'd been bit by fire ants. We all went to a local ER, but they refused to honor the notes because they weren't notarized. All we could do was go to a drug store and slather calamine lotion all over him until the burning went away.
DS#3 went to Cape Cod with a friend's family several years ago. We sent along the "please treat" letter, insurance card, etc. While there, he sustained a significant laceration on his foot. The letter wasn't accepted but they talked to me on the phone and I faxed permission to treat the laceration specifically, and they sewed him up.