We used this option several times in each park during our last trip. DD was three and one DS was 5. How is works is....first adult goes on the ride with any other children willing or able to ride, or alone. At the beginning of the line, where the CM is measuring kids, ask the CM for a child swap pass. They will give you one pass good for up to three riders. After riding they bring the Child Swap Pass to the waiting adult. The waiting adult can now enter the FASTPASS line with the Child Swap Pass and up to two additional riders.
*If the ride does not have a height requirement they will not offer a Child Swap Pass.
*Some CMs will require to see the waiting family members before issuing a pass.
*This also works with a Fastpass. Just do the same thing as above, but the first group enters through the Fastpass line with their Fastpasses and gets a Child Swap Pass and then the second group also enters though the Fastpass line.
Hope this helps!! Feel free to ask any additional questions!