Legal Question in Legal Ethics in Tennessee

While not required, it would be prudent to get a Special Power of Attorney for Child Care from the parents of the child for exactly the possibility you mentioned: If the child needs medical attention you -- and the parents -- don't want any delays while the medical staff argues over permission.

It'sa simple notarized "permission slip" that allows the parents to appoint you as their representative/agent for the purpose of taking care of their child. You may even be able to download one from the internet, they are simple. Just make sure it has any specific limitations (such as "no blood transfusions for Jehovah Witnesses" if that is an issue), and an expiration date shortly after the trip ends.