I had a friend with Ultrasonic hearing. No joke - he could hear the dog's training whistle, which we verified by blowing from another room. He'd scream. His parents discovered it when in a museum he could hear some motion? sensor or whatever. Just making the point, she may have a physical sensitivity or an emotional one. Dont rule either out.
Like another said, there are a number or tamer rides, but, I went last year with little cousins, and you can get very frustrated walking up to the front of a line and after 20 mintues, having to escort a little girl back to the benches.
You might also consider buying Ear Plugs. I found a Kit at Kmart ( I hate ringing noises). The kit had numereous types of plugs, and some block high noises, and others low, others block bursts, and others block all noises equally. North Consumer Products, Model 55000 has 4 types. or, try wherever they sell firearms.
She may truly need it, regardless of the cause. A magic bean, a smile, and a pat on the back might do the trick. My mother used guilt, also effective.