I was really afraid of rollercoasters as a little girl but when I turned 12 i found them really really fun. Just tell her that she might have fun-- go on a small one first.
Well there are two different Busch Gardens, BG Tampa and BG Williamsburg. If it is the one in Williamsburg, VA, I'd say to try either the Big Bad Wolf or Apollo's Chariot. They are probably the tamest and have no flips.