I'm thinking of kids age 4-7. Rank these in order from Most to Least scary/thrilling rides. And, "most scary/thrilling" doesn't mean in a good way, but in a "most kids aren't going to like that ride" kind of way.
I only have personal experience with SM, Soarin, and Kali - because my first trip to WDW was as an adult with my kids, and my son wouldn't ride anything scary. So, I don't have any perspective.
Space Mountain
Splash Mountain
Big Thunder Mountain
7 Dwarves Mine Train (speculative, obviously)
Soarin
Test Track
Expedition Everest
Kali River Rapids
Tower of Terror
My son didn't want to do anything that looked scary on our last trip (he had just turned 5). He did splash mountain once, but didn't want to do it again. Our next trip he'll be 7, and I'm hoping to start him off easy so that he'll enjoy them. I have a 4 year old daughter that will likely be fine with whatever I throw at her!