In the several years since they have been doing baby swap by giving out the paper "rider switch" pass, I have never been able to. However, folks here have posted that they have been able to and some CM's who post here say you can.
Most recently with my nephew who was just a hair over the 40" required for TOT. He was petrified and obviously too young to stay alone. The CM refused to give us the swap pass because he was tall enough to ride.
If a child is tall enough to ride but chooses not to - in my experience - you won't get a RS pass. You can however just use FP and accomplish the same thing since the parent riding second uses the FP line with the RS pass anyway.
TJ