I appear to have found (stumbled across, actually) an earlier child for William & Euphemia.
In 1822 at old Machar, Robert Fiddes marries Margaret Smith.
When Margaret Fiddes also indexed as Margaret Smith dies in Aberdeen in 1866 age recorded as 67, her parents are recorded as William Smith, farmer in Leochel-Cushnie and Effie Laing (both deceased).
The 1851 census shows Margaret Fiddes wife of Robert as 53 and born in Leochel.
I have not found a baptism for Margaret Smith, but it seems to me that "Effie" Laing is likely to be Euphemia.