This is how it worked for us 100% of the time during the span of our last annual pass (Sept. 2017-Sept. 2018) and all times previously. On our last trip, the castmember at FOP even thanked me for making it easy for her since I knew what I was doing. She told me she appreciated guests like me!
I'll give an example with 2 sets of adults and only 2 adults. Sept. 2018, we had grandparents, parents, DD5, and DS2 who's too short. You always have to show the too short kid at the line entrance.
Grandparents and DD5 have FP+ for Flight of Passage.
We all go to the castmember at the FP+ cue and request rider swap, showing her DS2 is too short.
She asks who's riding now, Grandparents and DD5 raise hands.
She asks who's riding later and I say "We (parents) are going to take DD5 again."
She scans parents and DD5's bands and off we go!
When they got off, we met back up, gave DS2 to grandparents. Then, parents and DD5 go back in FP+ line.
**I made us (parents) a FP+ for Navi at the same time and we took DS2 on that while they rode FOP.**
When it's just the 2 of us with the kids, we do it like this:
Husband and DD5 have FP+ for Mine Train.
I have FP+ for Pooh near the same time and ride with DS2.
We check in at Mine Train, show DS2 is too short, and they scan mine and DD5's magic band to return later.
What you said is exactly correct: "I’ve read say that only like mom and older kids need a fast pass and dad can wait with little one and get the rider swap." - Dad can only take two others back with him, so if you have more than 2 older kids, only two would get to ride again with Dad. I'm warning you, though, you'll hear a lot of "that's cheating the system" and "that's not fair!" on here asking about rider swap, but that's how it works. My son is only 2, but he doesn't deserve to miss out on his fastpasses just because he's too short. I think about my friend who's 5 year old is still too short for everything but Mine Train - she's paying big bucks for her Disney ticket, but saying everyone needs a FP for something she can't ride screws her out of the fastpass she's paying for because she's too short and can't ride alone. That's just my opinion. Like I said, we've been routinely since DD was 2 years old and never had an issue with Rider Swap. It's worked the same for us whether it was paper or electronic like it is now. The only difference is that there's an expiration time you'll see on your phone app for the rider swap now.
Oh! One more thing - you can only have one rider swap at the time. Last trip, we got a rider swap for Everest and DD5 decided she didn't want to go again. When we got to FOP, they asked about it and we said she decided not to go again, so they overrode it with the new FOP one.
