Hi DeShawn:
The weather in Orlando in late September usually hovers around mid-to-high 80 degrees fahrenheit. However, during certain cold snaps or night-time, the weather can really dip to the mid-60's. It's always a good idea to dress in layers in case the weather changes unexpectedly. Hurricane season, which starts in the summer, tends to taper off by beginning of October, but I would still be prepared for the typical Florida rain shower. Disney World is so popular, that there's no more true "off season," but anytime schools are in session (such as in late September) Disney World is less crowded. So you've picked a great time to go crowd-wise and weather-wise.
As for must-do for your daughter, hopefully these recommendations will be a good start to planning your trip.
Among many other things: at the Magic Kingdom, we always ride the Jungle Cruise, Peter Pan's Flight, Mickey's PhilharMagic, and it's a small world. At Epcot, we take in Spaceship Earth, Living with the Land, and tour the countries of World Showcase. Our must-dos at Hollywood Studios include Toy Story Mania and Muppet Vision 3-D. At Animal Kingdom, our family always rides the Kilimanjaro Safari and visits Conservation Station. You also probably want to enjoy a character meal, such as at Chef Mickey's or princess dining at Cinderella's Royal Table or
Akershus Princess Story Book Dining at Epcot's Norway Pavilion.
Enjoy your vacation.