In case of an actual life-threatening injury or emergency, something called "implied consent" comes into play. They will treat the child without any consent (the implication is that the child and his parents would not want him to die or lose a limb, so emergency care to keep him alive and prevent loss would be done).
You would be able to give him over the counter medications that you don't need a doctor for, but anything between "no doctor" and "life or limb threatening emergency" would need an authorization.
Besides the insurance info and parent's signatures, you want their contact information, the child's full name and birthdate, physician's name and contact info, any known medical problems, allergies or medications the child takes regularly, record of last tetanus shot (if possible). Notarizing the signatures would be good, but it would not be absolutely necessary (schools and day cares regularly get authorizations signed and none of them are notarized). The phone numbers to contact the parents are very important. A hospital or doctor's office can get verbal consent over the phone from the parents.
Places to get a form or a sample you can make your own form from:
-possibly the doctor's office has a form that they give to parents
-check on the web. I did a search on google.com with the phrase "authorization for emergency treatment" and "child" or "authorization for emergency care"and got a lot of sample forms.
You might also want the parents to sign something that says it is OK for the child to travel with you. My mom was asked for a form like like when she traveled with my neice.