I have found child swap to be different from attraction to attraction and even from CM to CM on the same attraction so ASK FIRST. Before you enter any line or use your FP ask the CM how they handle a child swap. Sometimes you get a slip that works like a Fast P where the 2nd parent and up to two others and ride through the fast pass line at any time that day. Other times the parent and non-riders wait in the exit and the second parent enters with one other person through the exit.
I have found however that if the children are tall enough but choose not to ride you may not be able to use child swap. Again it depends on the CM but generally speaking baby swap is made for familes who have children who don't meet the height requirement so they don't have to wait twice.
On rides when your youngest dosen't meet the height it should be no problem but the entire party needs to be present to get the babyswap pass.
Some examples from our Feb trip:
TOT, younger dd not to short to ride, tooo scared. CM says no as the entire party is tall enough to ride??
RnRc - dd and dh go into line and ask for babyswap pass - oops I am already on my way to get ice cream with younger dd and CM won't give Child swap pass without child LOL. Back to RNRC with younger dd - too short, yes on baby swap. CM gives us a baby swap pass, dh and older dd ride while younger dd and I get ice cream. Later I take older dd back on RnRc with me while dh takes younger dd to B&TBeast show.
Star Tours, ask about baby swap, entire family waits in line then before boarding me and younger dd wait at the exit. DH and dd ride and then we switch through the exit.
Honestly if your children are tall enough but choose not to ride you "may be" better off using Fast Pass.
I did find babyswap a great tool and very helpful just make sure you know in advance how the CM at each ride handles it.
Have Fun
TJ