Yeah I know how Fastpasses work my friend LOL, but that's not applicable considering their arrival time. If they arrive at the struce at 7:15 they still need to go through security, get on a tram and walk to the entrance of the park, whilst corralling 4 children, I would put them at the entrance around 8am, depending on the wait for tram. They won't be first in line for their fastpasses, their realistic pass time will be around 10-11 or 11-12.
Even if they arrive 15 minutes earlier like you suggested to OP, it would still put them at the gates around opening, while many people have been waiting already for an hour or more to be the first people in.
The truth is that I don't know how corralling kids adds to the experience. You make an excellent point. I also don't know how the M&F experience compares to the Toy Story Parking experience. All I know is my experience, on a Thursday, one week prior to the Thursday that the OP plans to attend. It seems to be a reasonable experience to work from, as the park hours and lack of EMH are identical.
We arrived at Toy Story Parking at 7:05, and were about the 25th car to be parked (I counted). We rode the bus and were through security and in the turnstile line (having used restrooms & rented a locker) by 7:25. We were through the turnstile by 7:35, and immediately entered the FP line. We got our FP at 8:03, with a return time of 8:55-9:55.
While there may have been some people standing in the line for an hour or more, there were only about 100-150 people in front of us in the FP line. I know, because I could see the entire line in front of me at 7:55, and I did a quick head count. (Obviously, that's a head count, not a count of actual park passes/FPs pulled.)
If the OP can plan well, there's a solid chance for an early FP for GotG, and then pulling a RSR FP very early on. It is, as far as I can tell, the best possible strategy for getting an FP for RSR before 3pm.