My situation was a little different at GotG (the child was about 5 at the time, so not old enough to be left alone), but the CM had one of ride and the other waited off to the side with the child. When the first person was done, the first person and child exited the ride, while the second person rode. We did ask about getting a rider swap at the entrance (he wasn't really keen on riding to begin with and we were 95% sure he would not want to ride it when we got to the front of the line), but the CM said that we could swap at the ride point because he was tall enough to enter the queue (so essentially, they would not issue a child swap for a child that was tall enough to ride).