Is it really a terror? I think it will depend on each kid who rides.
My oldest (now almost 14) has a love/hate relationship with ToT that goes back to the time he was 7. He rode it then with dh and loved it, then rode again with me, and was screaming so hard and loud I thought something must be really different from before! He then decided he had no desire to go back on. Next trip he rode it and loved it again! Then the next trip rode it and said he really didn't like it and woud rather ride RnRC instead. edited to add that he loved it after that and loves it now!
My youngest (now 10) is okay with the ride itself, but still says that the beginning of the ride (in the lobby, watching the Twilight Zone video, walking through the boiler area) is the scary part.
They do have a chicken elevator if he decides not to ride at the last minute.
To warm up for ToT (which is my favorite ride of any ride in the World) I agree that the TTA would be a good intro to the dark of Space Mountain, and things like Space Mountain and Haunted Mansion (both for the dark/spooky kind of atmosphere - if those are too scary, ToT will probably be). Really though, I can't think of a ride that is close enough to ToT to compare with due to the fall (well, Splash Mountain has a "fall" - maybe you should see how he does with that fall first?).