In high school I had to build a model roller coaster for a physics project, and then run a bunch of experiments on it. Our teacher said, "After this project you'll never look at roller coasters the same way again," and it's true. Other people see the coaster and think about their stomach dropping, how steep the drop is, if you get flipped upside-down, if they'll get sick; I think about acceleration through loops, the curvature of the turns, the design of the track and where it's most likely to break down.
I prefer smooth rides; if my head gets jostled too much, I get a headache. Of the three, Dueling Dragons had the smoothest ride. Hulk and RRR tossed my head around a bit more than I like, both were about the same. The music on RRR was fun. Overall I am more impressed with the coasters at Busch Gardens and Six Flags, but I knew US/IOA was not a roller coaster park. US/IOA have the others beat in theming and the other types of rides/attractions.
Also, don't forget about Revenge of the Mummy (indoor coaster). It is not as intense motion wise as the bigger outdoor coasters, but might be a little scary for younger children (the mummy is trying to kill you during the ride, and much of it is in the dark).
Flight of the Hippogriff is a very gentle kid-friendly coaster if you just want a small taste of roller coasters.
ETA: At RRR, we had to wait a few extra minutes to load because there was a child in the car returning that was crying (because the ride was too intense for him). I felt bad for him - he was crying so hard his family couldn't get him out of the seat. At Hulk there was a girl who waited until they got to the front of the line, then she saw the people coming back and decided she didn't want to ride. So you may want to do some less intense rides first and work up to the big coasters if your child has never been on a coaster before.