We were there last week also and used child swap three times.
There were four of us - DH, me, DD (9) & DS (5). I think child swap works differently if there are two parents but only one child.
DHS Rock n Roller Coaster - DD (9) and DH had FP+, DS (5) and I did not. We (all 4 of us) approached the FP line and requested a child swap. Attendant was visibly relieved as had just turned away an irate parent with a too small child so I think she was just glad that we weren't trying to get DS on the ride (he measured at 46" in bare feet before we left). She didn't ask who had FP, just asked who was riding this time. DH indicated that he was and he and DD scanned their Magic Bands. I was handed a swap ticket that looked like the old paper FP but said child swap on it. DD and I used this immediately after she rode with my DH. We showed it to the first attendant in the FP line who waved us through. When we got to the 2nd attendant, she said "scan your tags" and then disappeared and never watched to see if we did so. Because of that we could have kept the pass and gone again but I tracked her down and gave the pass to her.
MK Space Mountain - as DS was tall enough for this ride we all had FP+. However, he decided as we approached that he didn't want to ride. All four of us again approached the FP line and requested a child swap pass. We were given a hand written pass larger than the old FP. I rode with DD, using our FP+. Afterwards, we checked to see if we could change DS's FP to something else but it was later in the day and not too many options, plus it was cooler and wet out so standby lines were nearly nonexistent. So, DD and DS swapped Magic Bands and DH rode with DD using his Fast Pass and DS's FP Later that night DD and I returned with the child swap pass. At that time, the attendant asked where the child was, which surprised me. I said that he was riding a ride he wanted to go on with my DH (true - they were on the teacups). I think he would have kept us from riding but the standby line was only 10 mins at that point so he waved us through the FP line. No problems with the second attendant - we just gave him the pass. Just a heads-up though that you may be asked to bring your youngest both when you get the pass and when you use the pass.
AK Expedition Everest - this went pretty much like the Space Mountain experience except that 1. We were given a printed pass like RNR, and 2. We were not asked to present DS to use the pass, only to get it. Since DS had a FP for this ride also and didn't want to do ANY of the rides at AK by that point, we swapped bands again and my DD rode 3 times.
Hope this helps!
This is totally ot, but after reading this post I have truly realized what a PEANUT my DS is!! He's always been small and we always call him our peanut, but there is something about reading in print the heights of 4 & 5 yrs olds that are way bigger than my barely 40" DS who will be 6 in April!!!
I am just thanking my lucky stars he hit the 40" mark so he can go on most of the rides 
