I know the name Kerr could be correct, but this is what threw me - the name Bell.   I can't find her birth, or their marriage though there is a death for her in 23 Sept 1854 aged 30 (of decline) - that would have been 3 years after Samuel was born, and 12 months before William married Jane Grieve when Samuel was just 2 years old in the 1861 census.
This marriage looked likely, but the year would be wrong.
Name:   
Elizabeth BellGender:   Female
Marriage Date:   18 Jul 1845
Marriage Place:   Barony,Lanark,Scotland
Spouse:   
William MclachlanFHL Film Number:   0994192, 0994193, 0994194, 0994195, 1041481, 0994191
1851 census 
Name:   William Laughline
Age:   24
Estimated Birth Year:   abt 1827
Relationship:   Lodger
Gender:   Male
Where born:   Ireland
Parish Number:   622
Civil Parish:   Barony
Phillimore Ecclesiastical Parish Maps:   
View related Ecclesiastical Parish
County:   Lanarkshire
Address:   524 Duke Street
Occupation:   Labourer - Brickfield
ED:   5C
Page:   13
Household schedule number:   48
LINE:   9
Roll:   CSSCT1851_145
Household Members:   
Name   Age
John Bell   34
Margaret Bell   30
Margaret Bell   15
John Bell   12
Elizabeth Bell   10
Jane Bell   7
Mary Bell   4
William Bell   11 Mo
William Laughline   24
Elisabeth Lanchline   24
Samuel Lanchline   2 MoSource Citation
Parish: Barony; ED: 5C; Page: 13; Line: 9; Roll: CSSCT1851_145; Year: 1851
Source Information
Ancestry.com. 1851 Scotland Census [database on-line]. Provo, UT, USA: Ancestry.com Operations Inc, 2006.
Original data: Scotland. 1851 Scotland Census. Reels 1-217. General Register Office for Scotland, Edinburgh, Scotland.
It seems they were boarders, with the Bell family - her brother perhaps?
In 1841
Name:   Elizabeth Bell
Age:   14
Estimated Birth Year:   abt 1827
Gender:   Female
Where born:   Lanarkshire, Scotland
Civil Parish:   Gorbals
County:   Lanarkshire
Address:   Eglinton Street
Occupation:   Worsted Spinner
Parish Number:   644/2
Household Members:   
Name   Age
Robert Bell   40
Ann Bell   40
Janet Bell   20
Margaret Bell   15
Elizabeth Bell   14John Bell   12
Robert Bell   5
James Bell   3
In 1851
Name:   Elizabeth Bell
Age:   24
Estimated Birth Year:   abt 1827
Relationship:   Daughter
Father:   William Bell
Mother:   Mary Bell
Gender:   Female
Where born:   Barony, Lanark
Parish Number:   652
Civil Parish:   Old Monkland
Phillimore Ecclesiastical Parish Maps:   
View related Ecclesiastical Parish
Town:   Barrachnie
County:   Lanarkshire
Address:   Flocks
Occupation:   At Home
ED:   31
Page:   18
Household schedule number:   64
LINE:   16
Roll:   CSSCT1851_171
Household Members:   
Name   Age
William Bell   49
Mary Bell   49
Elizabeth Bell   24Mary Bell   18
William Bell   15
James Kinniburgh   19
Hmmmm I dont think that is right, as Elizabeth is in two 1851 censuses aged 24 in both 

Meanwhile I will look up your suggestions and get back to you.