It depends on your child, only you know your child & what they can or can not handle.
Last June we took our 3.5 yr old on everything, including Dinosaur, ToT, Splash Mountain, etc. He wasn't scare on anything.
He LOVED Haunted Mansion & ended up going on it 3x in a row! BUT I will share a story with you and the HM. My son was really excited to go on HM, but when we got to the cars you jump into, he said he didn't want to go (I think the darkness & noises scared him) I reminded him that it is just pretend (like I did before all rides) & that I will be right next to him & he can hold my hand. I also told him we can wave to the ghost we see. He then said OK & we did just that, waved to all the ghost. He LOVED IT!
But I knew that he would if he would just go on it but I know my child's fears & what he can handle.
Before going on ANY ride I explained the ride to my son & told him what to expect (dark, loud, etc) so he was prepared which I think helped alot.
Also we went to all the 3D movies. My son liked HISTA the best out of all of them. The only part he didn't like was the "snake" and he just closed his eyes for that one & was fine.
Like I said, you know your children. I know my son's fear theshold so I know what rides he can handle & which he can't (I would not take him on Alien Encounter even if he wanted to go on it) If you decide to take one or both of your children on a certain ride, just research the ride so you can explain the ride to them & let them know if it will be dark, loud, scary, etc in advance.
Good Luck