My now 9 year old, who is generally scared of everything new, went on every rollercoaster at Disney as soon as he was tall enough to ride 'em, and he loves them all. My 3 year old was just tall enough for Gadget's Go-Coaster in March in Anaheim (their analog of Goofini) and he couldn't wait to get on it again. Start small and see what she likes.
One additional thing I'll say is that Space Mountain was more frightening for my older boy because of the darkness and effects. BTMR, Everest, Primeivl Whirl, and even RnRC (no build-up to effects in line, I guess) were way less frightening for him than Space Mountain.
One additional thing I'll say is that Space Mountain was more frightening for my older boy because of the darkness and effects. BTMR, Everest, Primeivl Whirl, and even RnRC (no build-up to effects in line, I guess) were way less frightening for him than Space Mountain.