Jule, with what info you have so far, wonder whether this could be Robert's family (with the names of his children and the refences we have to Garron Hill/Muirkirk from Agnes' family?):
From 1841:
William Davidson 40, joiner b. Scotland
Jean Davidson 35 b. Scotland
Robert Davidson 15 b. Scotland
William Davidson 14 b. Scotland
Catherine Davidson 13
Ann Davidson 11
John Davidson 9
Elizabeth Davidson 6
Mary Davidson 4
N K (not know) 2 Weeks
Address: Muirkirk Garranhill
Jumping to 1851, is I think this entry for the family:
Wm Davidson 53, wright, b. Whithorn*, West Lothian
Jean Davidson 49 b. West Calder, Midlothian
John Davidson 19, wright b. Muirkirk, Ayrshire
Elizabeth Davidson 16 b. Muirkirk, Ayrshire
Mary Davidson 13 b. Muirkirk, Ayrshire
Address: Porter Lodge, Mauchline Ayrshire
*Likely Whitburn in West Lothian rather than Whithorn - Whithorn is in a diffent county, Wigtownshire.
One to keep on the back burner until you are hopefully able to confirm...
Monica