Hi Amanda!
We love you too! It is a joy to assist guests like you with planning their Disney vacations!
I brought my daughter to the Walt Disney World Resort for her first visit when she was just 17 days old! Since you are traveling in September, I would recommend that you dress the baby in comfortable clothes such as a onesie as September in Florida is still pretty hot. Also, don't forget the sunscreen! I used to bring a baby towel, a wash cloth, and a change of clothes and would refresh my baby in the restrooms, to cool her down in the summer months.
You may also want to check out the
Baby Care Centers located in each Disney Park. These are air-conditioned, full-service facilities just for mom with little ones. Here you can find comfortable changing stations, private nursing areas, and an on-site shop carrying diapers, formula, baby sunscreen and children's medicine. However, I would recommend that you bring your own as the store has a limited selection and you may not find the brand or item your little one is accustomed to. I would also recommend that you bring a clip-on fan for the stroller. If you don't have one, these can be purchased in the Disney Parks.
You guys are going to have a great time! Thank you for submitting your question to the Disney Parks Moms Panel!
Yael