My DNephew was terrified at 3.5 at DL last summer. He loves Halloween, vampires, the Nightmare Before Christmas so we thought that he'd love it. And in fact, he was the only one of the kids we took on the ride.
Well, the lightening and thunder in the stretchrooms scared him (I'd forgotten about it and didn't warn him). And he cried the whole time, poor guy (and poor DSister).
So, depends on the kid.... but do your homework so that you can explain what will happen.