Hi Lauren,
How exciting that you're planning a Walt Disney World vacation! I'm sure you're excited for the magical memories that are waiting for you. I'm so glad you stopped by with your questions. I've worked in the field of deaf education for over 15 years, so hopefully I can help.
First, I'd recommend looking at the Services for
Guests with Hearing Disabilities page, which has lots of helpful information, like when and where Sign Language interpretation is available and how you can request an interpreter for other shows and special events. There is also a downloadable Guide for Guests with Disabilities for each of the 4 theme parks with even more information. If there are specific accommodations that would be helpful that you don't see covered (for example, preferential seating at shows), just stop by Guest Relations in the park to discuss those needs and the possibility of a
Disability Access Service Card for your visit.
I'm not aware of any specific characters that know ASL, but I think you'll find that all the characters are absolutely amazing at interacting with your children. When you approach each character, I'd just let him or her know that your daughter is deaf. I also found this video taken over the summer that might give you an idea of what you and your little ones can expect when you meet the characters: www.aslnook.com/2014/08/disney-in-asl.
In terms of a couple practical tips, I'd pack plenty of batteries for your daughter's cochlear implant, as well as waterproof gear so that she can enjoy the awesome pools and splash areas in the parks and resorts.
Wishing you pixie-dusted planning, Lauren!
Beth