That could be me.
Yes we have been refused often because DS is an adult, tall enough. He is also clearly disabled and clearly can't be left alone outside a ride. We make it work and there is no reason others can't as well.
RS started before there was ever a FP system so families didn't have to wait in the STANDBY line twice. It is just a shame Disney has not kept up all their policies to match their newer systems in place. It does sound like there have been quite a few experiences in the last 6 mos or so of Disney requiring all to have FP+ to do RS at some popular attractions like FoP, 7DMT, BTM - with good reason. I won't be surprised if it includes Slinky Dog as well when it opens. Of course with Disney and so many cast members, procedures can change from shift to shift ... so the guests need to be prepared with Plan B.
How do we make it work? When some want to do SM, they do it and I do for my child .... I book my FP+ on a ride he likes and can go on. I don't mind doing that, because I want him to have a great day. But honestly if he were a little kid who wouldn't know any different, he wouldn't even get that his FP+ had not been used, or not used for him.
I just don't get all the angst of a young child not using all their initial FP+ - especially when you can book FP+ all day long. You can ride all day SB and using extra FP+ ... it's not like they only get 3 rides the whole day.
I was just there Easter Week = super crowded. We booked FP+ for 3 with no issues DAY OF. We got EVERYTHING at Epcot, at DHS we had no issues, at MK we just rolled all day long even getting 7DMT on two days.
We ended up using close to 20 FP+ per day so ... if DS didn't actually use one or two of his initial FP+, no big deal because he still did just about everything there was to do. So if it's a huge deal for
all the adults to ride
all the rides and not sit out one for their kid ............ do it, you can then just keep rolling that 4th so your kid can still go on lots of other rides using the FP+ lines.