I'll be taking my daughter in a few weeks and I'm having trouble figuring out who provides the drinking water and if they add fluoride to the tap water (water consumed during drinking and eating) or not.
There are also reports that the water at WDW tastes horrible, so you may want to consider bottled water aside from the issue of fluoride.
As a general rule, when traveling, if you are concerned about fluoride, I suggest using a trusted source of bottled water.
Also, unless you or your family suffer from severe fluoride allergies, drinking fluoridated water for a couple days probably won't hurt that much. Even so, I understand the desire to avoid it.