Good finds, whiteout7.
There are nine Croftheads in Lanarkshire, one in each of the following parishes
Avondale
Bothwell
Cadder
Carnwath
Crawfordjohn
Douglas
East Kilbride
Lanark
Stonehouse
Curiosity (and a few free credits) got the better of me.
Margaret Casey, 17, born Lanarkshire, Lanark, was a farm servant at Byre Town, Lesmahagow, in the 1871 census.
There is no 7-year-old Margaret Casey in the index to the 1861 census. There is a 7-year-old Margaret Cassey in Dalziel. She is the second surviving child of Peter Ca(s)sey (both spellings are used) and Mary Arbuckle, but her birthplace is given as Cambuslang not Lanark.
The children are Mary, born 1851/2 in Old Monkland
Margaret, born 1853/4 in Cambuslang
John, born 21 August 1858 in Old Monkland, presumably died young as he is not in the 1861 census
Agnes, born 20 May 1858 in Dalziel, presumably died young as she is not in the 1861 census
Andrew, born 24 June 1860 in Dalziel
Elizabeth, born 3 September 1862 in Cadder
Janet, born 20 October 1864 in Cambusnethan
Peter, born 2 March 1866, Cambusnethan
John, born 13 September 1868, Cambusnethan
Betsy, born 9 December 1870, Cambusnethan
Don't know yet whether any of this is useful information.