I've read lots of different posts on these boards about people's experiences with motion sickness. Some that have it all the time in cars, etc feel nothing on the ship and some that never deal with it in other areas seem to be very prone to it on the ship, so for your daughter's sake, I hope she is on the minimal experience end.
My best advice would be to book a room in the lowest decks 1 or 2. There are plenty of inside rooms to pick from or you could get an outside room where it would be a huge picture window where she could get natural sunlight and look outside (many claim that looking at the horizon helps return a sense of stability).
Also I would suggest a room mid-ship that seems the best bet for avoiding the feeling of movement.
Also I don't know if she is on any kind of medication for motion sickness but you may want to check with your doctor about what he feels would be best to ease her discomfort if she does have a bad reaction.