I have never used child swap for Splash or Star Tours, but I can speak to Space, Matterhorn, Indy and BTMRR.
Of those, Space is the quickest in terms of getting the second rider on because you enter through the exit and are typically placed right on the ride. If you use an FP expect that it will take about 20-25 minutes for the first rider and then about 10 minutes for the second rider to finish up. We usually kill time by riding Buzz (however all riders will need FPs for that, so plan accordingly) or playing with the big stone ball that is right outside the ride.
For Indy, we always just use the single rider line as it is typically 10-15 minutes faster than using a FP due to the fact that you go in through the exit and skip down to where they show the movie before the actual ride. Using SR typically takes 20 minutes from start to finish. As previously mentioned Tarzan's Treehouse and Jungle Cruise make great time fillers.
BTMRR child swap works like an extra FP. Get one FP for the first rider and then use the swap to send the other rider(s) through the FP line. Depending on how busy the ride is (this one gets progressively busier as the day goes on), it can take anywhere from 10-25 minutes per rider. Since we tend to ride this one as the last ride of the night I use the time that my DH is in line to visit the little pin shop in Frontierland. On the rare occasions that we do this during the day, we grab a frozen lemonade and walk down the Frontierland trail past the mechanical pirranhas or we watch the traffic on the Rivers of America (when it is not walled off for rehab).
Matterhorn is a strange exception to the other rides in that instead of picking up the child swap from the entrance to the queue, cast members want you to stand in line as a group until you are about 3/4 of the way through the line and you reach a green umbrella that is between the two loading areas. They will then have the people who are not riding exit the line and wait for the rest of your party to finish their ride. Then they will take you in through the exit on the Fantasyland side of the ride. The approximate time between you leaving the line and the first person finishing is about 10 minutes and the second rider will be done in 5-7 minutes. Due to the short time spent waiting, typically no distraction is necessary, but if your little ones are restless, watching the subs at the Nemo Lagoon could be a good choice.