If a child is tall enough for some attractions, you can do an internal rider switch with them. For the internal rider switch, everybody in your party goes through the queue and then at the load area, there is a bypass to the exit of the ride that the child not wanting to ride and an adult can go through and then wait at the exit. Once the party is done riding, the adults can switch off and up to three people (so the adult waiting and two others) can go back through without going through the queue again. They simply go back the way they came under the direction of the CM.
I've seen this done at a couple of attractions, but I'm not sure if all attractions have an exit hallway or the internal rider switch option. You would think they would all have some sort of "chicken" exit.
I think they have this system for children who are tall enough because it is faster for the family than having to go back in the queue (even the Fastpass queue). The rider switch card system works for kids who aren't tall enough to make sure the CM isn't held up checking heights when they are trying to load.
It also makes sense to why rides without height restrictions don't have the rider switch cards. You could probably bring the child through and do an internal rider switch.
Sometimes I wish WDW had a similar policy to
Disneyland where the first part of the party going through the queue for the rider switch gets a card that they exchange for the rider switch ticket once they get to where they collect fastpasses in the line, because this would cut down on people scamming for rider switches and make it easier for guests actually needing them.