Photo copy is a very good idea, if they are nervous block out any info that isn't really needed.
The other thing, assuming her parents are not going with you, is a medical release. It can save time if the child needs medical treatment if you have the power to take action if you are unable to reach her parents in the time of an emergency.
Good ideas! The following is a fairly standard form for activities and trips when a child is essentially under the care and supervision of an adult other than their legal custodian(it allows the appointed person to act as a parent/guardian for medical and any OTHER reasons that might arise (doo-doo occurs, think lost, runaway, abducted, legal issue, etc). Notarizing it would be nice, but it really isn't necessary:
The undersigned _______________________________, referred to as the parent and lawful guardian of _______________________________, a minor.
Parent acknowledges that said minor is authorized to take the following trip _______________________________, and to engage in all activities incident thereto.
Parent hereby appoints _______________________________, as loco parentis, and authorizes the rendering of such emergency medical care as could be undertaken by the parent. The parent hereby releases _______________________________ and its agents and employees from any and all acts taken in good faith during the trip.
Signed this ________ day of _______________________________, 20______.
In the presence of:
_____________________________
Witness
_____________________________
Parent