Hi Lyndsey!
Thank you for visiting the Moms Panel!
September is a warm month, traditionally, at Walt Disney World Resort, so you should be fine in sundresses. As it is your first visit, please watch that sun - it can be fierce - we often visit from the UK, and it is quite a different heat from ours. Sundresses should be fine for the evening too, but you may wish to have an extra layer (a light cardigan or suchlike) as the air conditioning can be very cool!
You will love
Disney's Grand Floridian Resort and there are plenty of amenities there including several shops which will provide all your necessities.
As this is your first visit, and I am also from the UK (and have returned from Walt Disney World only last week) please come back and ask me any questions you may have - especially about travelling, or anything at all about your resort, or dining, or how to plan your days - I would be very happy to help you!