Honestly, one of THE BEST trips I had at WDW was right after I broke my arm! I broke my arm in three places and had shoulder to finger cast on. This happened on Thursday. During the entire time they were casting me and everything, the only thing I could think of was "does this mean we can't go to WDW?!" Well, the doctors said that IF I went in a wheelchair and placed my arm on a pillow for most of the day (I could get up for on and off of rides) because the cast would take a long time to dry. I was able to see and go on everything I wanted to (I was 7), even Space Mountain, and was wheeled around all day, and you can bet I got some GREAT attention from the CM's! I even got a few signatures from the characters (though I don't think they do that now.)
Maybe sharing that kind of story will help her. I can't imagine not wanting to go to WDW. She should be able to wear a closed toe shoe by then, but maybe letting her know that there's wheelchairs available should she need it will take the anxiety away? Maybe pointing out that people IN wheelchairs (who can't get up and walk) still go and have a great time? (If she's gone before, maybe ask her if she remembers anyone in a wheel chair or on crutches, or with a broken limb and ask if she thought they were having a good time.) Is she old enough for you to have a "what if" kind of conversation with her? This works really well to dispell my aspergers son of some of his more "black and white" viewpoints. For example...."DD, if you had a friend, and she didn't have a leg because of cancer or an accident, would you tell her not to go to WDW because it wouldn't be any fun or she'd have a terrible time?" Or, "if xxx (insert best friend's name here) was planning to go with us to WDW but broke his/her toe (though frankly, I always go "bigger" with DS, like leg, arm) would you tell him/her that he couldn't go or that you didn't want him to go?" Often, when I put something in that realm, my DS really "gets it". I then say, well, if you wouldn't discourage xxx from going, why are you so convinced that you can't/shouldn't/wouldn't. Often, I don't even need to take that step with him anymore. I tell ya, it requires me to be quick to come up with analogies sometimes!
Help her prepare for possible/probable situations and then help her put it in perspective. Best of luck!