I don't think there is a formal rider swap since there is no height restriction. You could make your own, but it would take FP+ for all riders and I'm guessing your goal is to use rider swap in place of some FP+s.
If it's a fear thing, you can skip the stretching room (or one parent with the scared kid can). Just ask the CM at the door as soon as you enter. For my DD, that's the part that makes her not want to ride, so finding out we could skip the stretching room made a no-go a go.