Now I will tell you that not all rides do it the same way. Most do like everyone has mentioned, you show the baby, get the rider swap, one parent rides the ride, and the other uses the rider swap to ride it later and they get to use the fp line.
However, with that said, I know for a fact that Star Tours in the past has done it differently. Maybe there is someone with more recent information, but for our son's first trip (2007) when he was 18 months old he stayed in the line with us through Star Tours all the way up until the loading stage. Then I rode the ride and my DH stayed there holding our son. When my ride was done I was told to wait in my seat and my DH would meet me with my son. And my son and I could go on and exit the ride.
With that said, what actually happened was a little bit different, which I'm sure they won't do now with the change to Star Tours. But what they were able to do for us, was my DH met up with me and gave me our son and went on to another loading area. And the cm gave me the option of riding the ride with no motion and letting my son ride it that way. She said they were only able to do that when the line was really slow. Of course I said yes and my DS had a great time. Maybe that is one of the reasons why he loves Star Wars to this day. Afterwards he also got a certificate, for the year of Dreams and it said Jedi Logan on it. It was a great experience.
So be prepared for rides that might handle things differently. Just ask the cm.