I read this story as a kid, and I'd like to find it again! It was a story about a group of children with physical mutations like extra eyes, missing limbs, and so on, who lived in an group so isolated from other humans that they didn't even know 'normal' people existed, until one child became sick and non-mutants were forced to intervene. It was told from third person point of view of a preteen girl and her feelings about discovering she'd be considered a monster to the rest of humanity, when the community of mutant children were very supportive of each other and she was something of a leader. The one detail I remember well was the mutants all had names beginning with an "Em", something like Emellen, Emesszie, Emaarty (those aren't the actual ones), and when the non-mutants came one of them mentioned they were actually acronyms with the Em standing for mutant (MLN becoming Emellen, Emesszie = MSZ, Emaarty = MRT and so on).
I read this in the very early 80's, so it would had to have been older than that. It was collected in a large book of science fiction stories aimed at younger readers, and I think there were illustrations at the beginning of each story - I want to say they were painted, possibly in color, and they were small and above the title, but this may be my memory playing me false. I'm afraid I don't recall any of the other stories in the volume; one of them may have been about an underwater city. I'm pretty sure all the stories had children as protagonists.