We just got back from the Magic and my daughter had her hair braided on the ship.
I am not sure of all the times they have available but I know it is limited due to the length of time each person takes.
They started taking reservations on Sunday morning and they sold out right away. We got there early and we were second in line. We took the first reservation for Sunday (9:00 I think).
The cost is $2 for corn-rows and $1 for braids. It cost $22 for my daughter. She had corn-rows across the top/front of her head only.
Also, I had my hair french braided on Formal night. She fit me in between scheduled appointments. She only charged me $2.
I almost forgot... they also do hairbraiding on Castaway Cay.