One of the best and most authentic ways that the students in my classroom learn about people with disabilities is to have a student (or more than one) with disabilities as part of the class. For the last couple of years, my classroom has been the one chosen for second grade that includes students from the life skills room in our class. Last year we had three students with varying disabilities, and this year we have one second grader who has MR and seizures.
Our student joins the class daily for a portion of the Language Arts period. He participates as he can in the activities the rest of the class is doing. The children benefit from seeing this young man from day to day. He has made progress in his speech, which everyone can notice. He also uses a "talking box" called a Mo that can be programed to say phrases that are likely to be used in different situations. The screens can be changed to accommodate different settings, where different words would be said. Everyone has gotten used to the different sounding voice from the machine, as well as used to the types of things they may hear from this particular student.
This has been a good way for young students to learn to accept differences in people and in learning.
P.S. I don't remember the Seer being blind in The Giver (LibraryThing booklist of books about people with disabilities). Is that correct?