Hi Cynthia,
Welcome to the Moms Panel! The greatest thing about a Walt Disney World vacation is that everything is designed with families in mind. It sounds like a lot of work, but in my opinion this is the easiest vacation to take with little ones...simply because Walt Disney expected this and designed everything to accommodate them!
Anything attraction that doesn't have a height requirement will allow for you to take your younger daughter on, and believe me there will be plenty of things for her to experience!
If there is something that your five year old wants to experience, but a height requirement restricts the younger one, they offer a rider switch so one adult can wait with the smaller child while the other adult/child waits in line and rides, after they have ridden the adults "switch" and can re-enter the attraction with little to no wait. So both adults will get to experience the attraction, and your lucky daughter will get to ride twice!
Everything from the restaurants, to the restrooms are kid friendly and they have wonderful Baby Care Centers in each of the theme parks.
At this age, your younger daughter will not have to pay theme park admission, so I say go for it! The biggest argument I've heard is that "they won't remember it", but I can tell you the memories that
I have from my toddlers' vacations to Walt Disney World are priceless!
You can find some great information regarding vacationing with little ones
here.
Hope this helps, good luck with your decision!