When DD was 5 she got really scared on this ride. We have video of it, and you can hear her get worked up and start crying.
It is a combination of that age's imagination factor -- everything is real and can happen -- and good effects. You are right by the back of the evil queen, as she is looking in the mirror, and she spins around to face you as the hag -- and those eyes are CREEPY. This is accompanied by the soundtrack that is suspenseful and loud. You go through dark parts, with this music, and smaller ones worry about what else might turn into scary things. The chase scene has thunder, which frightens many preschoolers, and you travel under the hag who is pushing a giant boulder seemingly on the top of your car.
The next time, when she was 7, she enjoyed it and we rode it several times.
I think it is all in the age and temperment of your child. The smallest ones don't have the imaginative capabilities to read into the scenes to get scared, and the school age kids, at some point, develop the ability to understand the difference between real and imaginary fears, but many 4-5 year olds are just smart enough to comprehend the power of the story without the maturity to separate themselves from the scenes.