My twins went on Haunted Mansion when they were 3, 5, three times when they were seven. I remember all of them.
When the kids were five, my daughter was especially scared of the howling wolf you hear as you stand in line. I told her it was only Scooby Do, and she liked that thought. They've always been a little scared when they're in there. My daughter sits so close to me, she's practically wearing my clothes. There are no tears however. The intro is a little scary to them, but if you explain that everything is computers and not real that helps. They've never been scared enough that it ruined our trip or that they said they'd never go on it again. I would definitely recommend it, but you know your child more than anyone. Seeing how my five-year-olds reacted, I'd say go for it!
One attraction that did cause problems at five was Honey, I Shrunk the Audience. The kids saw that at three and were scared, but nothing too bad. However, when they saw it at five and were more "aware" of things, they came totally unglued and were screaming like they were being tortured. I carried them out of the theater clinging to me like vines. Then the rest of the trip they were worried that everything was 3D. They wanted to know before they went on every attraction before they would go on that it wasn't 3D. For me it was a simple question that they asked me every time, but I ran into another mom there who said it absolutely ruined her trip. Her children would not go on anything at all after Honey. However, they both wen on at seven and for my son it was fine. My daughter just takes off her glasses when she gets scared. They both love 3D Muppets and It's Tough to be a Bug. Maybe you should consider those first before the more intense HISTA.
Have a great trip!