I have been in a similar situation. DD 15 is afraid of most thrill rides. However, she really wants to try them, and once she does, she usually loves them (SplM, BTM, SpaceM, TT, MS, and Mummy at Universal). She has been to the front of RR 2 times, and then exited at the last minute. This year, I (being a bigger chicken than she is) went on it first with DD 13. I knew after riding it that the 15 year old would love it, so pursuaded her to at least go through the line. When we got to the front, she was ready to back out. I told her that was ok, but reassured her that she would be safe and could keep her eyes closed. She decided to get on. As they put the shoulder harness on tears started welling up in her eyes. She was terrified, but it was too late to back out. She kept her eyes closed through most, but got over her fear and went back to ride it again with her eyes open. It is now a favorite, but did I ever feel terrible sitting there seeing the fear in her eyes before we took off. I would let her go through the line with you, and then wait at the exit like the other posters suggested. That way you are only separated for a few minutes.