Hello, Elizabeth!  
Thank you so much for visiting the Moms Panel!  Sounds like you are preparing for a 
MAGICAL Walt Disney World Resort vacation.  How exciting you and your family must be!
You are more than welcome to bring along a backpack filled with treats.  Just keep in mind that glass containers are not permitted in the parks.  Also, you many bring along a cooler as long as it is a cooler which you can carry on your shoulder.  Rolling coolers are not permitted.  There are water fountains located throughout the parks.  Bringing your own snacks/food is a great money-saving tip that I love to share with our guests.  Most importantly, it keeps the little ones happy while waiting in lines.
My husband and I always bring along two backpacks.  One for treats and the other has sweaters.  I am not sure when you will be visiting, but there does tend to be a small chill in the air at night.  I also recommend bringing a small blanket and pillow.
You may also bring your own stroller into the parks.  If, however, you prefer to rent, you can just click 
HERE for some pricing information.  
I hope your vacation is filled with many new and 
MAGICAL memories.  Please come and visit us again should any new questions pop-up.  We are ready to help plan a fantastic vacation!
~Have a 
MAGICAL day!~