When we went for the first time in many years the first ride we took my just turned 6 year old nephew was Mission Space. I think this was his first time on a thrill ride. We went to Busch Gardens a few times when he was really little, but didn't ride much since he was so young. I hadn't been to Disney in quite some time, and thought it was more like Star Tours. Told him we'd just be going to space on a big screen in front of us. I really thought thats what it was. My mom said he wouldn't like it, but my brother and I took him anyway. When we walked in the capsule I thought, I don't know about this, but thought he'd be ok. As we leaned back he started screaming, and just kept screaming I don't like it, I don't like it the whole time. And the way you're in there, there was really no way for my brother or I to comfort him, other than to try touching his leg telling him it was ok and would be over soon.
That set the tone for most the trip, especially the first couple days. He didn't want to ride much of anything after that. We tried convincing him Test Track was just like a car ride, but he wasn't having it. Especially when we got in line and the car was above us on the track it was loud. He started crying and my mom took him out of line. We did convince him to ride Nemo which he liked (we did it a few times) and Figment, but that was it.
The next few days he just said he wanted to swim. We'd go to a park, and he wouldn't really ride much of anything. Things like Dumbo and Aladdin he was ok with because he could see it. Just said for the next 3 days he wanted to leave and go swimming.
Luckily for us we were doing a split stay, and lied to him and said our next resort didn't have a pool (Contemporary). We then got him on some rides, but didn't even attempt anything with any thrill. We really tried convincing him on rides, but he wasn't having it.
We went again when he was 8 and we had so much more fun. Although he still wasn't into thrill. Wouldn't do mission space of course, big thunder, splash, Everest, rockin roller coaster, 7 dwarfs, or tower of terror. Going again next year (he'll be 10 nearing 11) so hoping that he'll do all those. I think the only 2 he may still be on the fence about is RNRC and TOT, maybe Everest cause it looks high even though we tell him it doesn't go down a big hill.
I know he'd love all those rides, but no matter how much convincing he just won't do them. Now that he's a little older I can reason with him a little more. At our local park I tell him to just remember, if he doesn't like it he never has to do it again and that they only last a couple minutes.