Also keep in mind that some kids may just have sensory issues with going up and down or round and round and never like rides that do that, but rides that are flat (Small World) or can go up and stay up (Dumbo) won't bother them a bit. If you can figure out what it is about the carousel that bothers your child (the up and down-ness, the spinning etc) and if you notice she reacts similarly in other situations (like hills in the car) then you can simply reassure her that you won't take her on any "up and down rides" and if you choose your first few rides at Disney well, she will trust you and you will do fine.
If she seems scared, it is probably best to let her sit back and watch until she is ready.
All that said, they definitely do go through phases. My oldest DD is nearly 7 and has always been a ride everything kind of girl. But our last trip to DLR last month, she got scared and cried on Thunder Mountain, and now is not wanting to ride anything dark. So, we ride the out in the open rides only with her right now. If she can't see that the ride isn't dark, she doesn't want to ride it. On the same trip DD4 was riding ToT--every kid is different!