HO 107 1719 folio 435
Great Marlow Buckinghamshire
Joseph East 69 Great Marlow, Buckinghamshire, Head Married Gardener
Ann East 67 Great Marlow, Buckinghamshire, Wife Married
William East 31 Great Marlow, Buckinghamshire, Son Unmarried Gardener
James East abt 1827 Great Marlow, Buckinghamshire, Son Unmarried Gardener
Ann Tubb 26 Great Marlow, Buckinghamshire, Daughter Widow
William Tubb 4 Medmenham, Buckinghamshire, Grandson
Frederick Tubb 10 mths Great Marlow, Buckinghamshire, Grandson
Sophia Cox 17 Great Marlow, Buckinghamshire, Granddaughter Satin Stitch worker
HO107 1719 folio 453
Spittal Street Great Marlow Buckinghamshire
James Cox 44 Wycombe, Buckinghamshire, Head Married Married Farmer of 35 acres employing 3 labourers
Ann Cox 54 Great Marlow, Buckinghamshire, Wife Married
Considering the age of Sophia's parents the grandparents don't seem quite old enough?
Presuming Sophia did actually get married, Samuel if he never knew his father may have got his name wrong on his marriage, thinking he had the same first name as his own (that doesn't mean I think the Eton marriage is correct - there are two Harrises marrying there and the odds are it is to each other).
Sophia's father James was a farmer. He may have left a will on his death so that might be worth checking for. James had remarried by the time of the 1871 census and seemingly again by the 1881 census. This I think is his death registration
James Cox 80 March Quarter 1886 Wycombe volume 3a page 444
Matilda Harris the other child I think may have died young (but be very careful - Harris is such a common surname)
HARRIS, Matilda Death aged 9 December Quarter 1872 Wycombe Volume 3a page 299
Great Marlow parish registers are not on the IGI so it may very well be worth checking them as the children may have been baptised but not had their births registered.
In the end this family may prove a blind alley, but at present it is the only blind alley you have and with the intriguing family memory about the surname Harris it seems worth exploring if only to eventually eliminate them.
Regards
Valda