The September rain is all those little showers...UNLESS it's a hurricane band you are getting, then it rains alot. so it's give or take.
We really have not had to omany problems, but like you we like to just go for it!! DD stayed pretty dry with raincoat and she walked around. but at times she WANTED to get wet
USE THE BABY CARE CENTERS! honestly! they are a life saver in the heat. WE changed diapers, fed Autumn and sometimes she just wanted to run free and not be told to stay right next to us! haha!! (the one at AK is AWEsOME!!)
They don't sell Cheerios anywhere on property a lot of the time, so bring your own if they are a good snack for the little ones!! Autumn and Gabriel LOVE them!
LYSOL WIPES! if anyone is still in diapers, even with the siaper pad, you will WANT to wipe down those changing areas! we ALWAYS do! i even carry around my GIANT lysol can!!
WIIPES in general, they are a life saver for EVERYONE in the party! esp. if there is only one CS table open and it's DIRTY with ketchup and stuff!!!

we use them on everything and everyone!!
On Autumns gym shoe last yr, i had an alien green mickey head laminated and hole punched and put her last name on it and first name and our cell # JUST IN CASE!! we NEVER let her out of our sight and she was in the stroller 99.9 percent of the time, but we did not want to take any chances!
TAKE A PICTURE at the beginning of the day of what the kids are wearing and another one if they change clothes, b/c if something happens, you tend to forget what they were wearing.
okay for the fun stuff!!! no matter how clean the children are make sure you at least bring an extra shirt or you will be buying many t-shirts (spoken from experience!!!!). AND of course let them eat a mickey bar (wipes and extra shirt needed here!!) Autumn made a DISASTEROUS mess, but on video it's too cute!!
i also pack a swim suit so that the kids can run around in the POP jets at EPCOT and DTD and MK and AK!! they can get soaked in these locations!!

but it's fun and a good break from the stroller! oh and a good place to play when it rains!!