Definitely check the weather forecasts before you go and pack some basic raingear (like disposable ponchos and ziplock bags for your phone/electronics).
It is likely you will have at least one good, heavy rainshower during your vacation. Possibly more. It is early Fall in Orlando.
Perhaps check online for historical rain averages to see what the past trends have been. Some late Sept's have been better than others and with the weather, you never really know until it is on top of you. But by being prepared with a few things packed, you'll feel a bit more confident. Rain or shine, enjoy your vacation!