Just how old was Bones supposed to be in TOS anyway? He didn't look like a contemporary of Kirk's to me. I'd put him mid 40's. Kirk looked early 30's. I'd love to have seen McCoy's daughter but not in that episode. It was horrifying. Too bad she didn't show up in one of the movies.
I agree, I have always figured that McCoy and Scotty were contemporaries and about ten years older than Kirk and Spock. So, it would have been plausible for McCoy to have had a twentysomething daughter.