I would guess that a fastpass is better than a single rider line because it should be faster. It is also flexible because you usually do not have to back within the hour window, you usually have the rest of the day to use the pass.
With child swap, you show up to the entrance of the ride with your group. The CM will issue a pass to let you return to the fastpass entrance to ride again. It is good for the rest of the day and you do not have to ride right away. It is good for 3 riders total, I believe.
If the ride has fast passes, you will only need a pass for those riding the first time, do not get a pass for those that will use the child swap pass, it is not necessary
HAve a great trip! My boys will be the same age this trip...it is a great age!