She might fall asleep earlier than her regular nap time - all my kids tend to nod off as we take off because of the motion of the plane. Also, there is a bit of room at the back of the plane usually where you can stand if you need to rock her and the change table is in the toilets at the back of the plane so if possible, ask them to seat you up the back. It is also close to the toilets if your other DD and DS need to go midflight (which can get a bit tricky if they are walking up and down with those meal carts and your kids need to get to the toilet deperately).

Good luck with your flight and if it goes really well, make sure you come back to this thread and give me lots of hints and tips