Each ride handles it differently so the best thing to do is head to each ride and ask the CM at the entrance to the line how they do it.
Most rides now have a special fastpass they give for child swap. Its good all day for up to 3 people. Some rides have you swap at the ride and some others have you swap at the exit.
Sometimes if the child is tall enough but doesn't want to ride, they'll have the child go in the line and do the swapping at the ride itself.
What we normally do as a family of 4, with a DD who is afraid of all the big thrill rides, is at a ride we love like Rockin Roller coaster, we run and get 4 fastpasses (even though DD won't ride, we'll use her fastpass for another turn on the ride!) then we talk to the CM and get the special fastpass.
First, one parent goes with older DD on RnR with 2 fastpasses. Then second parent goes on RnR with older DD with last 2 fastpasses. Then one of the parents takes the 'special' parent swap fastpass and older DD gets to go yet again!
HOpe this helps!
