Child swap is cool and works great. It can be a little different depending on which ride you're on. You just need to tell the CM at the entrance that you need to do a child swap, and they will clue you in on the procedure.
Example: Space Mountain. We had FPs, so upon entering the FP queue and giving them to the CM, we mentioned we were going to child swap with DDs 5 and 10, who didn't want to do it. The Cm handed us a pass with instructions on it. All of us went through the queue together, then when we got down near the loading platform, they showed DH and the 2 kids where to go on the exit tunnel, where DD13 and I would pass by them on our way out. So, on our way out, DH and DD13 took the pass and went on through the FP entrance again and rode.
Indy: I heard a CM ahead of us telling a family with a baby that the kid to be swapped has to meet the height requirement to even be in the line. You had to make your own swap arrangements outside the line. DD5 was tall enough, but didn't want to go. Again, we were instructed to tell the CM halfway through the queue-- he let the swapper and kid go through a rope barrier and showed them where to wait by the exit. Then the swapper and the older, braver kid went back into the line at that halfway point, where the CM let them through, and rode.
BTMRR: DD10 chickened out, last minute, on the platform. She's just not a big roller coaster fan. For some reason, DD5 really wanted to ride. So the CM told our Chicken Little, "Just cross through the car to the platform on the left side, where you exit, and wait there for your mom and sister to come back and get off." I was fine with that, there were CMs keeping an eye on her, and she enjoyed standing there and filming us with the camcorder as we went past screaming! If we had all had to exit, I'm sure they would have let us all just go through to the exit.
Single rider lines are only on selected rides. In DL, it's only on Splash, now. Used to be on Indy, but not any more.
DCA: Single rider for most of the FP stuff, I think: ToT, Screamin', Mulholland Madness, Soarin', GRR, did I miss any? I don't think Maliboomer does? Oh, and TSMM! I hear that's the best way to do it!