Hi Rod
Welcome to RootsChat

This looks to be a likely 1851 census entry for the family - all the children down as born in Aberdeen:
John McKenzie 44, lab mill, b. Fordyce, Banffshire
Ann McKenzie 46 b. Alva, Banffshire
John McKenzie 19
Ann McKenzie 16
Alexr McKenzie 11
Magdalan McKenzie 8
Robrt McKenzie 6
Hellen McKenzie 2
Magdalin Donald 82, mother in law b. Alva, Banffshire
Address: Gordons Mills, Old Marchar
A few more children show for parents on IGI, in all cases with mother Ann showing as Mellis:
1. JAMES MCKENZIE Christening: 04 NOV 1827 King Edward, Aberdeen
2. WILLIAM MCKENZIE Birth: 18 JUL 1829 King Edward, Aberdeen
3. GEORGE ANGUS MCKENZIE Birth: 20 NOV 1836 Old Machar, Aberdeen
Monica