It depends on her age. If she is a minor the airlines generally will offer assistance. The best thing to do is call the airline and speak to a representative.
Yes, you do need to call the airlines and make special arrangements. Very old people ( like in wheelchairs etc) , handicapped, or kids need to be chaperoned by an airline employee. Then make sure you get her into their hands, and her dad is on the other end waiting. He needs to make sure the same thing goes on there, for her return flight.
I used to have my young daughter fly solo. Simply call Delta, and they ll tell you how to go about it. Know that they will impose an extra fee.... You should be able to walk the girl to the gate; a flight attendant will round up the kids flying alone, and have them board and sit together. The girl s dad will have to have proper ID to fetch her at the other end.
call delta and ask