Dennis:
I practice law in this area and get this question from time to time. The customs agents in the countries you visit will require you have some sort of document authorizing you to have custody of the minor child. U.S. Customs will also want this upon your return.
Child trafficking is a serious problem and all authorities are sensitive to the matter.
The specific requirements for this document vary from state to state but are usually simple. So, check with your local state law for the proper form of documentation. The child’s parents will usually need to sign it in the presence of a notary and witnesses, if necessary. Make sure the notary puts their seal on it as you intend to use it outside your state of residence. Frankly, it should be the responsibility of the child’s parents to give you the proper documentation to have the proper authority for the care and custody of their child. They should call their attorney to get you the proper documentation.
You also want to make sure that whatever document you use authorizes medical treatment and use of prescriptive and non-prescriptive medications. Is the child currently taking a prescriptive medication? Can you administer it? Some jurisdictions can be quirky here. Perhaps you will want to get the minor child an over the counter medication and administer it. The document should cover this point.
In addition to the child’s passport, you want a medical insurance card for them. Make sure the child’s insurance covers medical treatment outside of the country. You do not want to be on the hook for the child’s medical expenses if this need arises. I would also want to know if the child is allergic to anything. You should also make sure you have the parent’s contact information should the need arise.