Everyone goes through the line (regular stand by or single rider line- it doesn't matter). Everyone is stopped at the point where you see the test seats and the door for the exit, just before proceeding to the sorting hat area. You just let the TM know you're doing child swap. I went on the ride first so he let me proceed to the sorting hat area and told me to go the single rider lane (I guess since it was only me). DH was directed to the child swap room. When I got off, I easily spotted the room. The top half of the door is open and I saw some heads bobbing around so I figured that was it. It's pretty much on the loading dock area. If you find yourself following the crowd going down the stairs, turn around, you missed the swap room.