Since my original post I have found some additional information. There are two land records in Hants County for John Parr, each listing a daughter Sarah and son William, so that helps to confirm the link between those two, but they make no mention of his wife, so I'm assuming she had died by this point. Have you found any reliable sources that confirm Mary Anne Forrest was the mother of these children? I have transcriptions of the two records, if you're interested message me directly and I can send them.
I have also found a marriage in the records of Rev. Roger Viets in Simsbury, Connecticut for John Parr and Mary Ann Forest. This marriage date matches the date I've often seen in family trees online, but none of those trees ever indicated the marriage taking place in Connecticut. It seems Viets did move to Digby in 1787, so I wonder if the Parr family moved with him to Nova Scotia (if this was true it would mean Sarah was actually born in Connecticut, rather than in Nova Scotia, as is usually recorded).