In a nutshell... the attractions with height restrictions all have "rider switch" passes. When you get to the ride you would go to the greeter and let them know you are doing a child swap. They will give you the switch pass and advise you where your party can wait for you (some rides allow you to all wait the line the first time through, others have half the party wait at the exit). Once the first rider waits the line and comes back out the other rider will take the switch pass and go through the fast pass line...