I have worn glasses on the Rock 'N Roller coaster, space Mountain, Thunder Mountain, and every other ride at Disney World. I have never lost my glasses on a ride. Sometimes, I do wear a string on them just as a little extra piece of mind, but I have never lost them. Just make sure that they are secure on her face. If they are loose before your trip, take them to the place where you bought them, and have them adjusted to be secure.
One thing I suggest, though, if your daughter has an extra pair, make sure you bring them. I always travel with an extra pair of glasses and an extra pair of perscription sunglasses. One time on a vacation, I had a pair of glasses split in two, and I didn't have a spare pair, so we had call my eye doctor to get my perscription, and go to a store to get new ones. I haven't travelled with just one pair of glasses since. Sometimes, stores run sales that you can get two pairs of glasses for $99--it may be worth your while to invest in them.
Karen