My DS rode it when he was 6. He had done a lot of dark rides by that point, including Space Mountain and things like that.
He didn't like Dinosaur at all. It's dark, yeah, but if they've seen other dark rides by that point it's not that bad. In our experience, anyway.
The visual aspects of the animatronics didn't bother him, either. Nor were we aware of dinosaurs chasing us in the dark. Not afraid of anything we couldn't see, I mean. It's a frenetic ride, and we didn't even have time to think about something like that.
What he didn't like is that it is LOUD. The dinosaur at the climax of the ride is really really LOUD. I think the loudest thing I've heard on a ride.
Every kid is different, but that was our family's experience.