Kids are strange. It is subjective too- you never know what they are going to like or not like.
My DD loved POTC, Test Track, Spaceship Earth, Star Tours, and Soarin at age 3. Asked to ride them multiple times at age 3 and age 4. Age 5 or 6 she suddenly became terrified of all of those (except Star Tours), and of about every other thrill ride in the parks. We were surprised because there were things she had wanted to ride over and over in the past, that now she wouldn't set foot on. We barely got her on anything without heavy bribery until around age 8, and I don't think we got her back on most thrill rides (other than Soarin or Test Track which involved bribery every time) until age 9.
As for HM, I wasn't going to let her ride it at age 3 as I thought the stretching room would freak her out, but she had watched it on you tube and begged to ride, so I let her. She didn't scream cry or complain, but once we got off she said she liked the floating head, but she really didn't want to ride that again. Age 4- she wanted to ride it again, and she has ridden it every year since, and it never scared her. Oddly, she always loved Snow White's Scary Adventure-- we rode it at least 10 times on our trip when it was getting ready to be permanently closed when she was little. If you think something might scare them and are on the fence-- letting them watch a ride on you tube can help.
Also oddly, at age 3, although she loved the rides listed above, the ride that freaked her out most was People Mover. When we went into the dark area in space mountain she was convinced that we had somehow tricked her into riding space mountain and she freaked out and started screaming at us because she thought she was on a roller coaster and it was going to start going fast. We still laugh about that one years later! When they freak out about a ride, you just hold onto them and tell them it will be over in a couple minutes--- and in that case tell her it wasn't actually Space Mountain.
She wanted to ride Space Mountain when she was 5 because it was her friend's favorite ride and she wanted to go back home and say she had ridden it. We thought she would hate it. She was refusing to get on any of the other thrill rides including Test Track, and had refused SM in the past, and had been on BTMR at age 3 or 4 and hated it. To our surprise, she liked SM and has always ridden it since then. She totally refused to get on Big Thunder that year and for about 3 or 4 more years.
POTC it was that little tiny drop in the dark that suddenly terrified her. That is DH's favorite ride-- so it took toys or Mickey Head Ice Cream bars, Nemo and Spaceship Earth were too dark and scary. Test Track, she didn't like some of the inside part.
Our worst was when she was 10 or 11 and decided she was brave enough and wanted to ride RNRC. She was back to liking most of the thrill rides by then. We used FP for it. We got to the front and she completely freaked out and didn't want to ride. Less than 5 minutes later she says-- I really want to ride it-- lets go back and do it. The standby line was an hour-- I said no the line was too long -- she begged-- so we got in line. After over an hour in line, as it is about our turn to get on, she totally freaks out again, says she doesn't want to ride it and starts crying saying she wasn't going to get on. We knew she would like it- and that if we let her get out of it 5 minutes later she would be begging to get back in line. DH and I told her she was getting on this time and we would buy her something she wanted after-- she continued crying and the looks from people around us showed they thought we were mean awful parents. She pitifully said "I don't want to ride. I don't want to ride" as she pulled her harness down, and I could see the daggers people were shooting at us with their eyes. She screamed the entire ride. I think everyone around us must have thought she was miserable and we were the worst parents ever. We got off and as we exited the building she grabbed my hand and started skipping and said "Can we get in line and ride it again?
I would say start the 2 year old out with Small World or Peter Pan for dark rides. DD absolutely loved those at age 2, and there is nothing scary at all about them. If you go on a dark ride that has some type of slightly scary element first, you may have a hard time getting a 2.5 year old on ANY dark rides-- I know some people that has happened to. At EPCOT- Living with Land for some reason really seems to appeal to the toddler set--- a lot of kids we know, including DD, loved that ride when they were 2.
