Quite true and I agree that it is 'possible' there may have been earlier children. However, based on known information, I would suggest it slightly less than likely.
Parthenia Trevailer was baptised at Ludgvan 5th November 1696 to James Trevailer (married Jane Williams at Ludgvan 14th January 1686os) (There can be little doubt about her identity as she is the only one of that name found in the baptism records of the OPC and the entire England Births and Christenings 1538-1975 database on FamilySearch)
Parthenia's known siblings were Mary (bp. October 1687), William (b. 16th March 1689os), Elizabeth (bp. 1st October 1693) and Jane (bp. 27th July 1701) all baptised at Ludgvan. (ref Ludgvan Parish Register)
I should note also that James and Jane Trevailor both appear to have been buried at Ludgvan (19th October 1732 and 27th January 1738os respectively) so the family does appear to have been Ludgvan-based throughout.
William Trevailer (Parthenia's brother) married Synobia Phillips at Zennor 29th June 1722. He wrote his Will 19th September 1753 and it was proved at Ludgvan 4th May 1768. In that Will he mentiones the following:-
sister Mary wife of William Thomas (married 13th May 1716 Ludgvan)
sister Elizabeth wife of Joseph Bawden (possibly Joseph Boden of Perranuthnoe and Elizabeth Treviler of St Hilary married 27th April 1736 at St Hilary)
sister Parthenia wife of Robert Osborn
sister Jane wife of James Potter (married 13th November 1731 at Phillack)
wife Noabia was named residual legatee and Executrix
The Will was witnessed by John Curnow and James Nicholls.
So this establishes beyond doubt that Parthenia was indeed the daughter of James and Jane Trevailer baptised at Ludgvan in 1696. And it also suggests that, at least under normal circumstances, the marriage to Robert Osborn should have taken place at either Ludgvan or Zennor.
Now, as Parthenia was baptised in 1696 (and probably born the same year) and as the first known child was baptised at Zennor 26th June 1719 then the marriage should have taken place probably not before the end of 1702 by which time Parthenia would have age 16 and not later than 1719.
It is at this point that we strike the real dilemma.
Records for Ludgvan are actually very good and particularly from the period from around 1700 onwards and the last 10 years of the first register (1563-1711) shows no evidence of a marriage or any children for Robert Osborn. The next register is equally as good and quite legible and again there is no evidence of any Osborn baptisms or marriages prior to 1719.
Zennor is a different matter with, except for some scattered Bishops Transcripts, little that is legible of the first register prior to 1713. However the register is quite good from 1713 onwards and any entries that might be a little difficult to read can, for the most part, be found in the Bishops Transcripts. But once again, for the period 1713 onwards there is not one baptism recorded for a child of Robert Osborn until that of son John on June 26th 1719.
If you check FamilySearch (and cross-reference with the OPC) you will find five Osborn baptisms at St Just to father Robert. For three of those the mother is recorded as Mary and for a fourth she is Elizabeth leaving the remaining baptism with only the father's name recorded. But that particular event was back in 1706 and some 13 years prior to the first known child of Robert and Parthenia. And all of those baptisms can be account for with the marriages of Robert Osborne to Elizabeth Williams at St Just in 1706 and Robert Osborne to Mary Warren at St Just in 1711.
Finally - there is, I think, just one Parish that might be considered a likely abode for Robert Osborne and for which records prior to 1720 might be lacking and that parish is Towednack. Yet even so we do have BTs for 1711, 1714 and 1715 in the FamilySearch BT collection and I also have a copy of the Transcripts of Towednack BTs by Dr J Hambley Rowe which covers the years 1708, 1711 and 1714-1720 yet again there are no records for Robert Osborn.
My conclusion therefore is that it is highly unlikely there were any children to Robert and Parthenia prior to 1719 and that the marriage probably took place about 1718 or 1719 but may never have been entered into the register.
Remember also that during the period 1719 to 1740 there were nine children baptised to Robert and Parthenia.
CT