My older son was 4 on our last trip. We started with the tamer rides & worked up. First was Barnstormer, then the Mts in MK (Splash, BTMR & he was tall enough for Space Mt which he loved), then we tried Soarin' & Test Track. At DHS he did Star Tours. I decided against ToT. He rode Dinosaur & EE but was scared of both, so I wish we would have skipped those too.
He watched several shows on the Travel Channel so he was familiar with the rides & we also looked up some on youtube.
My younger son was 3 yrs old on that trip & 40" tall but does not like big rides so he did not try any of these. He is more of a Carousel type of guy