Jennifer,
Of course nobody knows your 9-year old better than you do, but here is my general commentary, from past experience.
I have been on the ride several times with nieces, and other kids of a similar age. I expected a fearful post-ride commentary, but they all seemed to love it! There are times where they go backwards, and seem to be somewhat precariously hanging out there, but I can't recall ever seeing any kids crying after the ride.
The encounter with the Yeti did not seem to scare either, but more thrill and excite.
Again, you know your kid best, but for what it is worth, I would say you are safe to try it out. If he/she is not coming at the ride with fear, and if they are more inclined to like thrill rides, I think you will find it is a higlight of the trip.
Either way, please send me a direct question from my bio page and let me know how it went! Would love to know for the next time I am asked!
Have a magical vacation!