Disney cannot prepare or process any food that you provide in their kitchens - this even applies to warming bottles or heating baby food. There are legal restrictions that prevent them from having unknown foodstuffs in their kitchens. It may, however, be possible for the Disney chefs to puree some of the food from their regular menu to suit your daughter's medical needs.
There is an entire department at Disney Dining dedicated to helping
guests with food allergies, food intolerances, other medical
and religious-based food requirements. Just send an email describing
your situation (at least 14 days in advance of your trip) to
SpecialDiets@DisneyWorld.com.
A cast member will get contact you about how they can assist you with your situation.
A great resource for you is a book called PassPorter's Open Mouse for
WaltDisney World and the Disney Cruise Line. This sells for
approximately $20 and is available through amazon.com and many larger
bookstores. Open Mouse gives incredibly detailed information for Disney
guests with any sort of challenge, including dietary issues.
You should also know that there are rooms at WDW with full kitchen facilities. These include the Disney Vacation Club villas and the cabins at Fort Wilderness. Blenders are available upon request, free of charge, at these facilities. For more information and pricing about these rooms, call Disney reservations at 407-W-DISNEY.