From a former Movie Rider/tour guide (aka: ME!)
If you are worried, get a youtube video and watch it with your kiddo. Talk with them, and explain that there will be a tour guide that disappears but will come back, a guy/girl with a pretend gun, maybe some warm fire, a pretend silly alien, and Mickey at the end along with Wizard of Oz. And that everything is pretend and NOTHING will hurt them. There are some very loud noises, and maybe they want to cover their little ears (gunfire in gangster and western scene, alien as well).
But most of all, NOTHING will hurt them! I've seen kids and adults jump at the alien scene(I never liked driving through there either), but once they see Mickey and Oz, everything is super happy. The movie ending can also be a little noisy, but most kids like it.
I will tell you though...I worked there a LONG time ago, when they still had the "Sorcery in the Skys" fireworks that went off directly behind the theater. We were on lockdown during the fireworks, but still had to keep the dang vehicles rolling. Let me tell you, driving though the aliens by yourself was not a happy thing to do!!!