There is a CM at the entrance to the line or somewhere along the way on most rides, otherwise when you load onto the ride there is Always a CM. You tell them you want to do the baby/child exchange and they will give you a, well something- I can't remember what it was- and tell you where to wait. One person stays with the child and everyone else goes on the ride (by stand by or fast pass, however you are doing it). They exit and take over with the child and the one who stayed behind then goes on (often from the exit, but there will be a CM to tell you what to do for each ride. If there is someone with you that can ride (a third adult or older child) they can go on with you (even if they rode the first time) so you don't have to ride alone. This is the coolest thing!