Hi! We were at MGM on April 4 and they were not using Fastpass for Lights Motor Action, I'm sure its busier in July so maybe they use Fastpass more regularly. Friends who visited President's Day week in February did use Fastpass, but their return time was still about 45 minutes before the show so it didn't let them slip in at the last minute for a guaranteed seat. There were only three shows of Lights Motor the day we were there and two conflicted with our ADR's, so I think we went to a 2:45 show. We went more than 45 minutes early, mainly because the wait times at the things we wanted to go were too long to make it back for the show, so we figured at least we could sit down for a while. They were still letting people in close to show time but honestly I have no idea where they sat, because the stadium was pretty crowded.
We did Fastpass for Indiana Jones, but that was sort of unnecessary our day. We had 12:30 -12:45 return for a 1pm show, and when we went just after 12:30 no one was being let in, and the line to get in the Fastpass entrance as probably longer than the standby entrance. That day I think you would have had no problem at all getting in 20 minutes ahead. Despite the crowds entering at the same time as us, we still were able to get second row seats, which my ds wanted.
We did Star Tours with the kids for the first time this trip and both enjoyed it. They are older than your kids, 10 and 7, but the 7 year old can be a bit of a ride wimp and had no problem with it. Neither dh nor I had any problems either--we both felt sick after riding Body Wars years ago and haven't done it since, but we had no problem with Star Tours. It is a great ride if your son is a Star Wars fan. We did Fastpass this ride, the line was probably 20-30 minutes when we went by around 10am and we figured it was silly to wait. The return time on Fastpass wasn't that far out, maybe 30 minutes or so, so we went to see Muppetvision 3D, used the bathrooms, and it was time to ride Star Tours (then we got the Indiana Jones Fastpasses right after, before our 11:15 lunch ADR).
Hope that's helpful.