Did a little research here is what I found:
A letter signed and noterized by both of the child's parents is needed. The document needs to state the child's name, the parents names, your names, where we are going and the dates. It must also state that the parents are giving permission to you to have their daughter travel out of the country with you. Basically, as much info as possible needs to be included.
If her father is hard to get in touch with, keep trying. If your wife has full custody, get a copy of that court order, too. Even though she has a passport, I'd put in an official copy of the birth certificate, too.
On the letter have the parent(s) photocopy their driver's license on the letter before it is notarized. Also have them put all their contact info on the letter (mobile,home,work, email and physical address) so they can be contacted if there is a question about permission to travel.
Take as much as possible to avoid any trouble or disappointment.