Combine them both!
Go through the single rider line (saves time). Whomever is not riding first will go to child swap, while the other waits. Once the rider exits, they go to child swap and the one who didn't ride gets on super fast! After that, exit, make your way through the locker area (on the right) and ask to go through the Castle Tour line. Everybody can go through that. When you reach the end of the tour line, they'll direct you to the exit.