My advice on parent/guardian swap (child swap) is to tell the CM at the attraction before you enter the line that you want to use parent swap and find out how they handle it. Do not wait until you are boarding. The baby/child unable to ride must be with you so everyone must enter or approach the line together. Some CM's have you all wait the line together, then the first parent and older child(ren) ride and other parent and younger child wait at the exit and then switch via the exit allowing the older child(ren) to ride with both parents. Off hand I think only Star Tours still does it this way.
Most rides now give you a rider swap pass. The rider swap pass works like a fast pass and allows up to 3 people total to return to that attraction anytime that day so the second parent can ride. You still all enter the line and ask to use parent swap. The CM will give the pass to the parent riding second, then he/she and the baby can leave to do something else while the first party rides and return to ride later the same day. I prefer the pass because the baby or child that dosen't meet the height requirement dosen't have to wait the line for something he/she can't ride.
Just to clarify, the purpose of parent swap is to allow a family with a child who dosen't meet the height restriction to enjoy the attraction with only "1 wait, rather than 2" not to expeidate the wait. Sometimes it is quicker and more convient to just use fast pass instead of the swap.
Easy to use, difficult to explain!! Just remember to ask every time as it can vary even on the same attraction depending on the CM working the line.
HTH
TJ