So much misinformation in a single thread...
Yes, there is a special type of Fastpass, called a Rider Switch pass. It is only available at attractions with a Fastpass queue and a height restriction.
Complete details here.
In brief, you get them from the greeting CM at the entrance. You need to show the child who is too small to ride, and they may be measured to be sure. Having a child who is tall enough but simply does not want to ride does not make you eligible for a Rider Switch pass.
The first group of riders enter through the Standby line (or the FP line if they have FPs), while at least one adult waits with the child outside (or goes to another attraction, etc. - no need to wait (*). Anytime after the first group returns until park close, you can use the Rider Switch pass for up to three guests (so the adult who waited doesn't need to ride alone) to enter the Fastpass line.
At attractions that do not have a Fastpass queue or a height requirement, or with an taller child, as the CM what can be done. Most attractions have what is called a "chicken exit" and swapping in or near the load area may be possible.
As for Star Tours, yes, it appears that they have brought back the RS passes, although not necessarily every day. Again, ask the greeting CM to find out.
* Sometimes at Mission: SPACE, the Rider Switch passes are given at the exit to the game area/gift shop, which means you must be waiting by the doors there to get the RS pass.