The first thing you would do is to tell the CM at the entrance to the attraction, before you enter the ride line (either the Fastpass line or Standby line), that you want to use the child swap feature and find out how they handle it; each ride or CM could/might handle it differently but for the most part they are all handled the same, with the Rider Switch Pass (only if it's a Fastpass attraction).
They do not let children that do not meet the height requirement through the line (except on Star Tours).
The parents and the child must approach the CM together. They must see the child who doesn't meet the height requirement. The CM may measure the child if they are close to the height requirement. The CM will then give you a rider switch pass (if the child doesn't meet the height requirement and if the ride has a Fastpass line) and the parent that is riding second can then leave with the child and to do something else while the first parent/party rides. They could wait outside or wait at the exit of the ride (which is usually in a store) or do a different attraction. Meanwhile, the first parent/party can ride however they've decided; either Fastpass, standby or single rider, if available. Then the second parent returns, at their convenience, to ride via the Fastpass line with the rider switch pass and they can ride with up to 2 other guests; the rider switch pass is good for up to 3 guests total and is good all day.
The only ride that has the child swap at the loading dock is Star Tours. So you would all wait in the line together.
Child swap is meant for those who are not tall enough to ride so if your child is close to the height limit, they may be measured. Also, if they are not tall enough to ride, they will not let the child enter the line.
If your child is tall enough to ride but doesn't want to, then you can still create your own form of swapping by getting a Fastpass for each parent and then taking turns. Works just about the same as getting the rider switch pass. Or you could ask the CM if there's a way you could possibly swap at the ride itself or at the exit to the ride.
The purpose of child swap isn't a "head of the line pass" or to eliminate a double wait for a family that can't ride together but to shorten the wait for the second parent as much as the Fastpass line can and to make it more convenient by letting you come back later if you need/want to with up to 2 other guests to ride with you so you don't have to ride alone.
The parent riding with the rider switch pass does not need a regular Fastpass; the rider switch pass is all they need to enter the Fastpass line. It takes the place of the regular Fastpass.
Hope this helps.
