At most attractions, child swap works like this:
Everyone goes through the line (regular or express). At the loading area, whomever is going to ride first gets on the ride. Whomever is going to wait is directed to the waiting area, which is near the exit. Those that rode get off and go to the waiting area and swap. Those that did not ride can take any that want to ride again with them to the loading area (short walk). They then get on the ride in less than 5 mins.