There is a marriage record for a John Kinghorn & Euphan Dervar 5 May 1775 @ Dunfermline, Fife.
Their children were: David c:10 Dec 1775 @ Tulliallan, Perth, [ This is a 17 yr difference on the date you gave so maybe there was another David born to this couple? ]
Helen c: 13 Jan 1782 Margaret c:21 Nov 1779
Both girls were born at Dunfermline. This information I got off the familysearch site but I also have aset of vital disks & found more on them but interestingly there wasn't a David recorded also Euphan's maidian name is spelt Dewar: