Hello Mary!
I found a marriage on the IGI for a James Mitchell and Mary Cameron 18 Dec. 1807, Old Cumnock, Ayr. This is where James (the son) was supposedly born according to the 1881 census.

Thank you very much for the dates of death for James and Helen. I didn't have those.

I have the following children listed for Robert Kerr and Martha Steel taken from FamilySearch (no documentation).
Mary Kerr B. 28 Apr 1809 Kilmarnock (seems quite a bit older than the rest, but not knowing when Robert & Martha were born, it's a toss up.)
Mary Carr c. 10 Aug. 1823 Dundonald, Ayr
William Gibson b. 8 Mar 1821 Dundonald, Ayr
Emma Campbell b. 20 Aug. 1825 St. Quivox and Newton, Ayr
Helen (you have)

Now for Agnes Mitchell McConnell and James. I don't know the story about why she came here. However, James McConnell was married in Scotland to Isabella Pearson in 1864 and they had 6 children. Only the two oldest survived. John C. b. 1864 and James P. b. 1865. Isabella died a month after giving birth to Annie who died 8 days after her birth.

Here are the names of James and Agnes' children,
Agnes Mae b. July 17, 1878
Robert Mitchell b. Dec. 25, 1880
Albert Edward b. Dec. 12, 1881
Walter Francis b. May 10, 1883
Charles Allen b. April 12, 1885
David Barbour b. Dec. 8, 1887

I would love to have a copy of the picture of Helen Kerr Mitchell and will gladly pay for it. I have one of Agnes standing by a bush. Also one of James, Agnes and children Robert, Agnes, Albert and Walter sent to me by another relative.

My husband is the grandson of Walter. He stayed in La Salle county where I live.

Looking forward to hearing from you.
Sandi