A detailed description I can't provide. However, we recently went with our 9,6,3 year olds. Our 9 & 6 year olds thought it was silly and weren't scared our 3 year sat very close to me and though was "concerned" he didn't scream or cry but was too young to appreciate the humour.
You initially enter into a room in which the lights go out and it is pitch black for 5-10 seconds. The room then begins to stretch with the ceiling either going up or the floor going down depending how you perceive it.
You then enter the actual ride area where you sit in your "doom buggy" and go on a tour of the mansion where you see a variety of ghosts and goblins. No one jumping out and screaming at you, nothing squirting on you etc. Just ghosts, goblins & noises doing normal every day things in the Haunted Mansion, including dancing in the dining room.
If your 5 year old is tentative, scares easily you may want to wait a year but you know your child best. Our kids hated Tough to be a Bug -- it freaked them out, but Haunted Mansion was no problem for them.