eal401 - Have you thought of contacting the airline in advance:explain the problem and ask if they can help in any way. I can't promise that this will work, but might it be worth a try?
On my airline (actually my employers' airline, they just lend me the aircraft for the day) There are a couple of seat rows that are held back specifically for those in your position: The seats are not at exit rows, but one row back: The seat immediately in front, adjacent to the exit is not fitted. These seats are used both for the disabled and also for very tall pax. We also sell exit row seats at a small premium, but as you say, if your daughter is under 16, that's no use to you. What might be worth trying for is one seat in the exit row, and two in the row behind?
I hope that this works for you...