When we first started doing single rider with our DD, I went on first, DD went on second, and DH went third, so she would always know that one of us would be at the end. Now, we don't pay attention to who goes in what order. If she's on before us, she waits at the exit. If we're on before her, we wait at the exit. We've been doing this for 4 years now without an issue. She feels all "big kid" and we get practice letting her have a little more independence.
Now, as Super Anxious Mom (I need that on a cape with a bugler announcing my arrival), I made her prove to me, multiple times, on each ride with a Single Rider line, that should could manage the whole process without my assistance--so she had to show me that she could follow the CM's instructions without my prompting, could get into the ride vehicle, could secure herself appropriately without assistance, could ride safely, and could disembark appropriately. Once she proved she could do that on each ride, we would let her do Single Rider.
Now, at 13, she's an old pro at it, and rolls her eyes at me when I ask if she's comfortable doing single rider on her own if I'm not in the mood for a particular ride. DUH, "Mom, I can do it, I'm not a little kid!"