Approach the CM at the entry to the ques and tell them that you need a child switch pass. They will need to see the child who is not riding and hand the pass to the adult who is waiting with the child. Once the other rider(s) have come off the ride, you (and one other person with you) can use the exit que to use the switch pass. You can actually use the pass at any point during the day. You don't have to use it right then.