It does seem now that we do have the same common ancestor Andrew Boddon.
You'll find the census document on Scotlandspeople for 1841 showing the family.
Search:
Andrew
Bodden (sic)
age 35
Kirkcubright
Carsphairn
This will show Robert aged 6 months
For the McMillan Grandparents in 1851
Search:
Robert
Bodin (sic)
age 10
Kirkcubright
Kilpatrick Durham
It cornfirms the correct family as the sisters names are the same with the correct ages
You'll need to buy the previous page for the grandparents details.
You can also get the Will/Testament for Robert McMillan (cost £5) from Scotlandspeople
Search:
McMillan
Robert
1853
This confirms the link to Andrew Boddan (sic) surgeon, it makes quite interesting reading IMO.
It's also likely that Andrews parents could be the ones mentioned above by Matt in the James Bodden family grave 1770-1839 as the year range seems correct.
As far as the correct spelling of the name goes, my ancestor gave the spelling as Boddon on her marraige cert, although the grave stone states Bodden, take your pick

Tom