We just went with Grandma (my mom, 73 years old) and she just wanted to stay at the resort and enjoy it. Since you asked about express passes, I am assuming you are not staying on site, so your experience may be a little different. Are you sure the grandmas want to spend three full days in the parks, or might they enjoy a resort day or two?
My husband, two DDs (8 and 2) and I had 3 day passes, we only spent about 8 or 9 hours a day in the parks (took breaks in the daytime), and we were absolutely exhausted at the end of each day. I don't feel that we went at a leisurely pace, either. We moved quickly and still did not do everything (although we did repeat several rides).
I imagine you'll be fine without express passes in September. We really only needed them for about 5 or 6 rides at the end of May (but they sure were nice to have for those rides).
As a previous poster mentioned, the child swap is very well done. I couldn't imagine it being easier or more efficient.
Be sure to spend some nice time in Camp Jurassic. It was our family's favorite place in the park. It is especially nice in the early morning and late evening when it is less crowded. The atmosphere and theming are fantastic.