Her post asks if they can all walk in together (this means the line) and then 2 ride while the other 2 wait at the front of the line for the other two to finish and pass the baby off. She was asking how it works. No, it doesn't work that way, the non-rider must wait outside the line. I don't understand where the confusion is.
This is exactly how I read the OP as well-- whole family enters the line together and then literally swap at the loading area.
I believe this is further backed up by the fact that when another poster said the infant cannot enter the queue the OP said "So Disney doesn't do baby swap". That certainly implies that she thought the infant needed to enter the queue for a swap to take place.