Well. It works the way rider swap is supposed.
Say my spouse and I are traveling with an infant child. But we both want to ride Slinky Dog Dash rollercoaster. We can wait in the regular line, or we can get fastpasses.
In this caae you have a fastpass, so let'a say my hypothetical self and spouse also have fastpasses. All people entering through the fastpass line require a fastpass, regardless of if using rider switch. You go to fastpass line, scan in, one adult waits with baby, one rides ride, then you do it all again for the second adult. If you're trying to find out if there's some sort of baby watching person at the rider swap entrance, sadly, no lol.