John and Florence Ivy are not notable for their part in the narrative of Discworld history; their little tragedy has been repeated across the Multiverse infinitely. In the story of Discworld they are only an excuse for a leading character to be away from home, and they fade from view when their task is done.
The Great Witch must not receive her invitation to the infant Princess's naming ceremony; the story says so, everywhen. On Discworld the story1 wins, every time. To this end, Granny Weatherwax was called out to assist Mrs. Patternoster when the pregnant Mrs. Ivy was kicked by a cow and began to miscarry. The four-page story of Granny's management of the crisis (in Carpe Jugulum) demonstrates one of the author's best story-telling tricks and also explains, memorably, what witches on Discworld actually do.
As usual, Granny makes the best of the situation2, but her invitation to the palace is stolen and the story nearly repeats itself.
1 As modified by Esmerelda Weatherwax, of course.
2 Not to Mrs Patternoster's satisfaction as she believes John Ivy should have the chance to choose but, as Granny says, "What has he done that I should hurt him so?" - i.e. make him choose between his wife and his child. She makes the cold, hard choice, by which she will stand 'til her dying day, but John and Florence can always try again, whereas the alternative is his losing both.