We used child swap quite a bit in December 04 and agin this February 05, we always were given the child swap pass rather than having everyone wait in line and routing the second riders back through the exit.
When you get in line, either the FP or Standby lines, alert the CM that you want to use child swap. They will give you a "child swap" pass, looks like a FP but has child swap (or non rider pass) on it. You can return at your convenience that day with up to three people total. You only get one CS pass per party unlike with FP.
In my experience the non riding child did have to be present to get the pass. Since your not entering the line the non-riding child can stay in the stroller but all the CM's asked which was the non-rider.
Then one parent can take the non-riding child to do something else while the rest of the party rides. At some point, that same day, there is no time restriction, the parent who didn't ride the first time can return and ride with 2 others. Its nice as the big brother/sister can ride with each parent and neither parent has to ride alone.
Things we do while waiting, Space Mountain, non rider does TTA, Buzz or teacups. BTMR, Splmt, non rider does the nice play area under Splash, or rides the railroad. RnRC, TOT, non rider does BnTB or yummy ice cream right at the exit. Test Track, or MS, MS has a great toddler play area or head over to Mexico for the boat ride.
Cellphones or walkie talkies are ideal for meeting up after the ride, even with FP sometimes it takes a while.
TJ