Hi Bruce,
I have indexed the entirety of Campbell records for the parishes of Fortingall/Kenmore/Weem/Killin.
I have identified and separated the Fortingall families of Rannoch and Glen Lyon, and have been assembling the family trees with secondary sources including Throth's website.
John Campbell and Janet Stewart's children as Forfarian identified:
Duncan, baptised 29 February 1756 - Carie on south side of Loch Rannoch
Alexander, baptised 16 July 1758 - Camghouran east of Carie
*Donald , baptised 20 May 1761 - at Inverhadden - but mother is Jean Stewart - so may be different family or recording error
Ann, baptised 17 April 1763 - Camghouran
Archibald, baptised 31 May 1766 - Inverhadden
Donald, baptised 21 August 1769 - Inverhadden
John, baptised 18 July 1772 - died 8 Oct 1861 at Fortingall, Kirkton
Finlay, 14 November 1775 - Learan
I took a look at the Campbell trees that have John settling at Anson, Carolina. I do not believe that tree fits this family, as Finlay's father John would have been born 1728 ish and this John has no son Farquhar or daughter Catherine.
The name Farquhar does not appear AT ALL in these parishes for Campbell - but it DOES however appear in the Argyllshire and Aberfoyle records. Maybe Finlay was not related to John/Farquhar or maybe it's just the wrong Finlay.
Going through Donald Whyte's Dictionary of Scottish Emigrants to the USA, it appears the the vast majority of Carolina settlers were Argyllshire or lowlanders. Perthshire almost exclusively settled New York. This is just from what I have seen in sources and mapping out families and may not always be the case.
That being said - I have seen a number of migrant families immigrate while leaving some children in Scotland, so be aware that does occur, I am just not sure that is the case with John Campbell and Janet Stewart. Do you have a death date with age in your sources for John and Janet?