Hi, Matthew!
What a special time with your grand-daughter -- you are generous and thoughtful grandparents to provide such a fantastic experience for her!
If the child is traveling without either of her parents, you will need to have a Minor Authorization Form completed before arriving at the port terminal. Disney Cruise Line (DCL) requires at least one parent to sign, and of course, you'll need proof of her citizenship, too.
The form is located online at DisneyCruise.com under
My Online Check-In and will also be mailed to you with your other cruise documents. The only caveat I will give you is to check the regulations of the country you're visiting (Bahamas) in order to be certain that you won't need additional documentation. The laws of the ports of call may be different than the rules for DCL.
I hope this helps you plan, and let me know what else I can do to help. Have a wonderful day and a magical vacation. Bon voyage!
Andi