Perhaps this is them in 1871 and 1881...
1871 at So Muirhead, Cumbernauld.
James Rowat, 56, Cotton Weaver, b Cumbernauld
Mary (wife) 56, Cotton Winder, b Kilsyth.
William 25, Limestone Labourer, b Cumbernauld.
Margaret 23, Cotton Weaver, b Cumbernauld.
Agnes 17, Cotton Weaver, b Cumbernauld.
Alexander 14, Cotton Weaver, b Cumbernauld.
Note: James Rowat is listed as Jams Rowat, so if you were looking for James perhaps this is why you couldn't find him.
1881 at Main Street, Cumbernauld.
James Rowatt [Spelt with two "T"s] 66, Cotton Winder, b Kilsyth. [I think he was actually born in Cumbernauld]
Mary, wife, 66, Cotton Winder, b Kilsyth.
James, 7, GRANDSON, b Cumbernauld.
In 1881, none of their children are living in the same house.
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Dates of birth (or christening dates ?)of some children. These are on Scotlands People, OPR records.
James Rowat 15 April 1838.
Mary 16 Aug 1840.
Robert 26 Mar 1843.
William 3 Aug 1845.
Margaret 8 Aug 1847.
John 19 May 1850.
The above are all Old Parish Register records.
Agnes Rowat 1854
Alexander Rowat 1857
These are also on Scotland People but are not OPR's. All these children were born at Cumbernauld to James Rowat and Mary McKinlay.
Hope this helps a little bit,
Brian