The North Farnham Parish Register shows that Thomas Osborn's second wife Margaret was Margaret Canterbury and had married John Smith on 9 Feb 1725/6.
Births for Thomas Osborn's children John, Mary, and Anne are recorded in the North Farnham Parish Register. Son John's death is also recorded in the register, although with a clearly incorrect date. The death of a Mary Osborn is given as occurring on 29 Nov 1726. Given that John Osborn was born on 26 Nov 1726 and the Richmond Co., VA Court Order books clearly show that Thomas Osborn remarried about Feb 1726/7 to Margaret, widow of John Smith, it is clear that the Mary Osborn who died 29 Nov 1726 was Thomas Osborn's first wife Mary (Jones) Osborn. It would seem most likely that Mary had died from complications of the birth of her son John three days prior.
Thomas Osborn's daughter Mary's birth is recorded twice in the North Farnham Parish Register. Once as daughter of "Thomas and Mary" Osborne and the second as daughter of "Thomas and Margaret", both entries with the same date of 24 Nov 1727. Mary had died by this time though and Thomas had remarried. Mary's mother is clearly Margaret (Canterbury).
It is less than certain who the mother(s) of daughters Margaret and Prundence is(are) as births or baptisms for these daughters haven't been located. At the time of his death, Thomas Osborn was married to a woman with given name Frances. Frances remarried to Cuthbert Harrison. Prince William Co., VA Court records show that Mary, Anne, and Margaret were boarded out shortly after Thomas Osborne's death, while Prudence evidently remained with Frances. Prince William Co., VA Will Book C on page 235 contains an inventory of the part of Thomas Osborn's estate belonging to Mary, Ann, and Margaret. Pages 194-195 though contain and allotment to "Mrs. Osborne for her thirds and her childs part" ("Prince William County, Virginia Will Book C 1734 - 1744" by John Frederick Dorman). From this, it would seem that Prudence was daughter of Thomas Osborn's last wife Frances, while Margaret was likely daughter of Thomas Osborn's wife Margaret (Canterbury).
According to unverified information from Barnes and Harrison genealogies, Thomas Osborn's last wife, Frances, was Frances Barnes, daughter of a Matthew Barnes of Charles Co., MD. This seems quite possible, as Owen Jones, father of Thomas Osborn's first wife Mary, was also from Charles Co., MD and the Jones family and Barnes family were intermarried themselves.
As a last note for Barnes researchers, I will note that many Barnes genealogies include a Frances Osborne married to a Matthew Barnes and this Frances Osborne is reputed by many to be daughter of Capt. John Osborne of Somerset Co., MD and his wife Attalanta. This is impossible though as Capt. John Osborne's will shows that he just had a daughter Martha and a child unborn at the time the will was written. Records of Accomack Co., VA (which borders Somerset Co., VA) show that the unborn child was a son given the name John Jenifer Osborn. So Capt. John Osborne and Attalanta had no daughter Frances.