We used Child Swap for ToT and RnR in July. It worked beautifully. Our kids were big enough, but not quite brave enough for these two, and we didn't want to miss experiencing them. With both rides, we used Fastpass for all four of us. Then we returned in our Fastpass window, went through the Fastpass queu and usually along the way we'd find a CM and we'd let them know we were child swapping (by having all four tickets, our kids still had the option right till the last moment to ride if they wanted to). One parent was then ushered to the remainder of the line up, and the other was sent to the gift shop with the kids. When parent #1 finished the ride, they went to the gift shop and got Parent #2. Parent #2 took the 'back route' from the gift shop back to the line and the CM let us right into the line to get on (no additional waiting). It was excellent....kids were happy, we were happy - system worked!
Good luck! Oh - and we had no 'tickets' for the child swap. Just used Fastpass, and told the CM's what we wanted to do.