Kurt Vonnegut's "Harrison Bergeron" has characters with intellectual disabilities. In the society in the story, anyone who is intelligent is strapped with handicaps that hinder their minds from processing and those who are "slow" are left to their own devices. Harrison's mother Hazel, is naturally "slow" and she does not understand simple concepts nor can she remember much of what has just happened.
You might try "Flowers for Algernon" by Daniel Keyes. Keyes worked as a high school teacher for developmentaly disabled adults. Keyes drew upon thies experience and combined it with his love for science fiction to create a story about a mentally disabled man whose IQ is tripled by an experimential operation. It received the Hugo Award for science fiction stories, Keyes later expanded it into a full length novel.
I think of "A Christmas Memory" by Truman Capote, one of my favorite stories. The narrator is a boy of seven whose best friend is a distant cousin of sixty-something. He has lived with her for as long as he can remember. Capote writes:
"Other people inhabit the house, relatives; and though they have power over us, and frequently make us cry, we are not, on the whole, too much aware of them. We are each other's best friend. She calls me Buddy, in memory of a boy who was formerly her best friend. The other Buddy died in the 1880's, when she was still a child. She is still a child."
The narrator's "best friend" is clearly a woman with intellectual disabilities, but their relationship is one of mutual love. The story tells of the joy and comfort they bring to each other.