I think it really depends on the child. You know your child better than anyone else. Let her take a look at the drop, see what she thinks of it. Be honest with her about it. You may want to see how she does on some of the tame dark rides, as parts of Splash are in the dark. Twice, my sisters have taken their kids on Splash the first day of the trip, and they've gotten scared and were apprehensive about going on rides they couldn't see.
My sister thought her son, who was four at the time, would love Splash Mountain, and she told him there would be a chicken exit before the big drop. Big mistake. The entire ride, he kept saying, "I want to get off. I want to get off." He was afraid of most rides the rest of the trip.
Karen