Traveling With Kids - Using the "Baby Swap"

The "baby swap" allows a group to ride the more thrilling rides at the parks in shifts when some members of the group are too short (some rides like Space Mountain and Star Tours have height minimums to ensure that the restraint systems work properly) or are simply too young (i.e. an infant or toddler) to ride.

The "baby swap" is when a portion of your party gets in the attraction queue. The rest of the party stays with the child(ren) that can't or doesn't want to ride while the first group transverses the queue and the ride. When the first group is done, the second group enjoys the attraction while the first group stays behind with the non-riders. It's kind of ironic that the baby isn't really swapped so much as the adults are.

The advantage of the "baby swap" is that the second part of the group doesn't have to wait through the entire queue. That's a big advantage when the lines for the ride are over an hour or more.

The "baby swap" is handled differently depending on the attraction:

Magic Kingdom

Alien Encounter - Both groups have to wait through the entire queue. Effectively, this attraction does not really have a "baby swap".
Big Thunder Mountain Railroad - The group that is staying with the child(ren) waits at the attraction exit, then waits for the first group of riders. When the first group is done, the second group walks up the exit to the loading area and is seated on the next train.
Space Mountain - Everyone in the group must go through the entire queue. This is a real pain, because you're still not allowed to take a stroller. Only when you get to the loading area is the part of the group that is staying with the child(ren) directed to exit (temporarily) down the last exit ramp. That part of the group, along with the child(ren) waits at the attraction unload area (inside the building), then waits for the first group of riders. When the first group is done, the second group walks back up the same exit ramp to the loading area.
Splash Mountain - The group goes through the queue together. When you reach the loading area, the first group rides. The second group is allowed to walk over the flume and wait on a few nearby benches until the first group is done.

Epcot

Body Wars - The group goes through the queue together. When you enter to load the simulator, the group that's waiting first walks through the row of seats to the benches at the exit. After the first group rides, the cast members will allow the second group to load from the exit before the other guests are permitted to enter.

Disney / MGM Studios

Twilight Zone Tower of Terror - Like Space Mountain, in the Tower the whole group stands in line together until the very end, then part of the group exits and waits for the rest of the group. When the first group of riders is done, the others enter through the exit and tells the cast member that they are completing a "baby swap" and they are allowed to ride.
Star Tours - This is handled just like Body Wars. The group goes through the queue together. When you enter to load the simulator, the group that's waiting first walks through the simulator to the exit. After the first group rides, they switch places.

Animal Kingdom

Countdown to Extinction - The group that is staying with the child(ren) waits at the attraction exit (to the left of the attraction building). Then, when the first group of riders finishes up, the second group goes to the cast member at the attraction entrance and is escorted to the pre-show room, so you get to hear the "set-up" for the attraction. Unfortunately, that also means that you have to wait through the last portion of the queue from the pre-show room to the vehicles load area. It's good to hear the pre-show, but having to wait through so much of the queue is a pain.
Kali River Rapids - The group goes through almost the entire queue together. Just before you walk down the stairs to the loading platform, the group that is staying with the child(ren) stands off to the left-hand side and waits (this is just at the top of the loading platform stairs.) When the first group returns, the groups switch off.

Some things to keep in mind when you're using the "baby swap" strategy:

ALWAYS let the cast members at the entrance and loading areas of the attraction know that you're using a "baby swap". The CMs can help you make things go smoothly. They are also up-to-date on how the "baby swap" is to be done.
Everything is handled by the cast members in a very cordial and efficient fashion. They are clearly very used to working with guests on the "baby swap".
Riders are never expected to ride alone. This can work to your advantage. Since each parent is permitted to ride with another guest, an older child or a third adult can enjoy the attraction back-go-back.
The "baby swap" is not advertised. You won't see signs in the queues that you can do such a thing. Even so, you should never feel guilty for trying to get away with something...the "baby swap" is allowed by Disney to enhance the enjoyment of the guest. Take advantage of it!