It's harder to be more definite on the 1851 about the other children. They are working away from home/family so difficult to say whether it's the right McKenzie. For example, this is the only John McKenzie showing born in Contin in 1826 +/- 2yrs:
John McKenzie, 24, labourer, b.Contin
Address: Milltown Brach, Petty Inverness
And for James, searching 1828 +/- 5yrs:
James McKenzie, 25, farm servant, b. Contin
Address: Graney, Rafford, Moray
And Catherine, searching 1832 +/- 5yrs:
Catherine McKenzie, 17, house servant, b.Contin
Address: 27 Kings Street, Inverness
Hard to say if they are the right ones but I am searching by first name and birth place only to pick up on any spelling variants of McKenzie.
As for 1841, cannot find any of them as yet....I'll keep looking

Regards.
Monica