Hello Claire and thanks for bringing such an important question to the Disney Parks Moms Panel.
In my opinion there's nothing more magical than taking a holiday to Walt Disney World Resort with a large group of family or friends; you are sure to have an absolute blast all together!
The first thing I would suggest; on entering any of Disney's 4 Theme Parks seek the advice and general guidance of a Guest Services Cast Member who will be only too happy to discuss the individual needs of the disabled child in your party, plus there's no need to worry about bringing along any proof of disability.
Each Guest and their entire party are treated as individual cases; so even though it isn't usual for so many Guests to be included on a Disability Service Access Card, by speaking with a Guest Services Manager the possibility is you may be able to enjoy Walt Disney World Resort all together as a group of 12.
For your further information you may wish to check out these useful links -
Guests with Disabilities &
Disability Service Access Card - Frequently Asked Questions.
If I can help sprinkle any extra pixie dust onto your 2016 holiday plans to Walt Disney World Resort, please pop back and ask me another question.
Have a great Disney day.
~ Jane