My DD first rode a week or two before she turned 6 and she was scared. We had to even get out of line to go to the restroom (her nervous reaction) and then get back in line. Right up to boarding, I think she really wanted me to say forget it. But we rode and she loved it. It's mainly tame till the end and then you just go fast. It's 64.x MPH which slow compared to a real highway but feels faster in the small vehicle and convertible. But you never feel like you're going to fly out or loose your seat. Even the curves are banked so you don't lean into the person beside you.
I think what scares the kids is being outside the building and hearing those cars ROAR around the building. You can't see them but the sound is something. When you don't know what it is and being young, I can see that being frightening.
A couple of years later we were there and I noticed a little girl about 6 ahead of us in line, fighting tears, Mom trying to tell her it would be alright. I sent DD over to talk to the girl to tell her how cool and fun it was and that she was scared her first time too.
So I'd suggest listening on the outside and if it bothers her, try to explain that it's just the cars on the track like BTTR and maybe talk to some other kids her age that have done it. Even go to the exit and watch the kids coming off. I've seen other kids really nervous getting on but have seen only grins once they get off.
Have fun...Sharon