I wouldn't recommend rnr for a child scared of the dark. It is VERY dark in there. I would wait for another trip to attempt it.
I love rollercoasters, and I love RNR, but, this is the ONLY rollercoaster that makes me nervous! And, I've been on ALOT of rollercoasters! I think it is the extreme speed that it starts out at, then the darkness of it all. I really hate the beginning, where it feels like you are being shot out of a cannon!