I have the same 6 birth records you have, but have found no trace of them, or Margaret, in any census.
I have a death certificate (1887) for Margaret Martin, widow of John Martin, with parents John Leitch and Ann Morrison. The certificate has her age as 76, putting her year of birth @ 1811. Which sort of helped to bridge the gap between John and Archibald. The witness/informant of the death is stated as Donald McCorquodale, a son-in law, but I can find no record of any relevant marriage.
So there’s nothing that actually links Margaret Leitch to any of the other six children, other than common parents’ names. I bit disappointing, really. If you have anything to fill in gaps, that would be great.
As you know, my line goes from John Martin/Margaret Leitch through Flora Martin/ James Brown and Ann Brown/Duncan Ormiston. Where are you in all of this?