Howdy Rose! I'm so excited for you and your family!!
The most important thing I would recommend is that you have comfortable shoes for everyone. There will be a lot of walking and comfortable shoes are a must.
Given the time of year you will be at Walt Disney World, be sure to have sun tan lotion, sunglasses and/or anything else you might need to cover sensitive skin.
I don't know if you typically travel with a stroller but your younger children may need it. I'm repeating myself but...there will be
a lot of walking. Be sure to take a towel or blanket with you to the Parks if you take a stroller. Use it to cover the stroller when you are in an attraction to prevent burns when little ones sit down in the stroller.
One of the most important suggestions I can make is to be sure to pack your camera and to consider
Disney's PhotoPass +. Disney has PhotoPass Photographers positioned throughout the Parks ready to capture your magical moments. Photographs will be taken throughout your vacation and will be sent to you on a CD with all of your pictures. PhotoPass + includes the ability to customize your photos and includes pictures from select attractions and Dining experiences. You will need to pre-order PhotoPass + prior to your vacation. You will receive a voucher in the mail that you will need to pack with you as you must have it in order to pick up your PhotoPass + card. PhotoPass + has become a must have item for us whenever we visit Walt Disney World.
Please let me know what other questions you have as you continue planning your vacation. I'm so excited for you!!