Hi BoBo,
There were no Sherrets in the 1851 or the 1861 Fettercairn Census but the 1861 Fordoun Census has this:-
Mains of Glenfarquhar
Wm Sherret 64 Farmer 200 acres b. Fetteresso
Isobel Stephen 65 Wife b. Fordoun
Isobel Sherret (U) 33 Dau. b. Fettercairn
Elizabeth " (U) 28 " b. "
Elsie " (U) 26 " b. "
James " (U) 26 Son b. "
Jane " (U) 24 Dau. b. "
Robert Collie (U) 22 Ploughman b. Bervie
John Linn (U) 15 Horseman b. Dumbarton
Chas.Robertson (U) 18 Cattleman b. Dunnottar
Mary Stephen, Lodger 6 Scholar b. Fetteresso
You'll notice that the enumerator has entered the maiden surname of William's wife. Elsie is another form of Alice, but I couldn't make out if the seventh person is Jane or Jessie.
I reckon the family to be as follows:-
Wm. Sherret married Isobel Steven at Fordoun on 14 Dec 1822. Children:-
Isabella 17 Jan 1825
Alexander 24 Aug 1826
Sarah 15 Aug 1828
Joseph 26 Apr 1830
Elizabeth 26 Feb 1833
James 20 Nov 1833
Alice 20 Nov 1833 (Twins)
Jessie 10 Jul 1836
Jane 08 Jun 1838
William 13 Jun 1843
I couldn't trace the 1823 William you mentioned, but there's a William born 21 Apr 1821, christened 16 Nov 1822, to Robert Sheret & Margaret Christie
at Fettercairn. Perhaps, as you say, the first William died at birth and was never christened.
Isles.