Married 1842 Falkirk > son born abt 1844 America > son born 1846 America > back in Scotland for 1851 census.
That is a rather quick turn around. Maybe they didnt emigrate. Maybe Roderick had employment as a ships carpenter, and the children were born onboard, wherever the ship was. Might not have been America, but an American colony.

Cannot find anything in America.
Later census gives James McKenzie born in
New York and also in
Canada.
ADDED -
From newspaper obit for Roderick MacKenzie junior who died in 1915
He was born in Canada in 1847, where his father (a carting contractor in Ross-shire) had emigrated shortly before his son was born. Who knows how accurate that report is?