I am a mom of 4...no mulitples, but I have traveled several times to WDW with infants.
It is very hot in Spetember, and when we had our infants it seemed to work better not to have them in carriers/infant carseats...too great a risk of over heating small bodies and dehydration, etc. We did use the clip on stroller fans to keep air circulating around them and these worked really well.
Keep in mind that rain happens often and fast in WDW at that time of the year, so each time you leave your stroller you will want to cover it or you will risk having to place an infant in a soaking wet seat. Another reason to perhaps avoid having the carriers in the parks or you could have wet carseats and wet strollers. We use a cover by Graco (available for single or double strollers) and it workds so well. Have to say, we did forget to cover once and it did of course pour. We bought a beach towel and placed that in the stroller and that worked in a pinch. But they are $20 a piece in the parks and with three infants that could add up fast!
Over the years we have had many a meal in various restaurants with an infant in a stroller (LeCellier, Coral Reef, Chef Mickey, Crystal Palace, GF...even in the castle and that is a tiny restaurant). Probably the only place you will have an issue will be the restaurant in the Land in Epcot, because no strollers are allowed in that building at all.
We have found Disney to be very accomodating of infants too small to sit in a high chair. Our youngest is 2, so the last trip we ate in a restaurant with an infant in a stroller was July 2005. As well, I never had much trouble eating with a front carrier or sling...even learned to nurse baby and have my meal and it was actually very relaxing even for me to have a nice restaurant meal. Buffets always worked well since there was less time delay and it was easier to stagger eating if need be.
I hope you have a fun trip. I know it isn't always relaxing traveling with babies and kiddos, but what an adventure it always is and oh the memories
