When growing up I always remember my parent's having a "permission slip" for whomever they were leaving us with. After having kids myself and going on trips I knew I had to do it as well in case there was an emergency and my kids needed medical assistance. I found one like this online and then added my own details to it. I am not sure if this is required anymore by insurance companies but I didn't want to have anybody stuck in the position where my children could not get medical assistance. Hope this helps!
Parent's Medical Consent and Release Form