Hello Kathy,

I think this is your Thomas Watson, in the 1851, census index, for Muirkirk.

Watson Thomas age 47y, Muirkirk,
Watson Jean age 17y, Muirkirk,
Watson John B. age 11y, Muirkirk,
Watson Margaret age 5y, Muirkirk.

and almost next door,

Mcdonald Archibald, age 27y, Muirkirk,
Mcdonald Janet, age 21y, Muirkirk,
Mcdonald Maigret, age 1y, Muirkirk.

Janet Mcdonald, is Thomas Watson's daughter, who was named Janet Maitland Watson.

Hope this helps,
Jonn.