A little nervous here as I have posted very little on Wikitree. I noticed that most of your sources are Family Search and Ancestry so I went in a different direction.
What I'm listing are results from a quick and dirty search of the National Records of Scotland via scotlandspeople.gov.uk. This information is taken from transcriptions as there is a cost involved to open the original documents.
Found only one Ayrshire birth for an Anne Watson: Watson, Ann Hepburn, b/bap 03 Mar 1803 at Ayr to James Watson / Catherine Richmond.
Didn't find any Ayrshire births for a William Milton but did find: Milton, William, b/bap 25 Jun 1794 at Paisley (west side of Glasgow), son to William Milton / Janet Clark. That puts this William about 31 miles from Sorn.
Wouldn't put a lot of faith in the above births, they are a shot in the dark. I don't know how long you have been searching the Scots and you may already know this but I'll mention an often used traditional naming pattern for a couple's children that has helped me a little in the past: first son - named after the father's father; second son named after the mother's father; first daughter named after the mother's mother; second daughter named after the father's father. Neither of the above births meet this criteria.
The marriage is recorded as 30 Oct 1818 at Sorn and as 30 Nov 1818 at Mauchline. Could be a transcription error.
I did find solid b/bap dates for their four children: MILTON, George, b/bap 04 Apr 1819 at Sorn, son to William Milton / Anne Watson; MILTON, John, b/bap 30 Aug 1820 at Sorn, son to William Milton / Anne Watson; MILTON, William, b/bap 15 Sep 1822 at St Quivox (about 13 miles from Sorn), son to William Milton / Anne Watson; MILTON, Margaret, b/bap 00/00/1824 at St Quivox, daughter to William Milton / Anne Watson.
Since Anne is noted with William at Kilmarnock in the 1841 census I tried to find a death between 1841-1851. Prior to 1855 Scotland's women tended to keep their maiden name. So a I did a search for both Milton and Watson. No hits when searching Ann Milton however I did find the following for a Watson. WATSON, Anne, d. 10 Jan 1845 at Dailly (about 30 miles from Kilmarnock).
Nothing but the children's information is solid but it does introduce you to another source. Hope some of the above was helpful.