Are your dates correct? 1851 Census for Canada West (now Ontario), actually taken in early 1852 (ages given are as of next birthday):
Beckwith, Lanark County:
Leech, James, farmer, b. Canada, farmer, aged 26, married, Church of England
Leech,Ann, b. Canada, aged 19, married
Leech, Samuel, b. Canada, aged 2
Leech, Margaret, b. Canada, aged 7 (suggests a second marriage for James)
They are living next door to:
Leech, Samuel, farmer, b. Ireland, farmer, aged 50, married, Church of England
Leech, Jane, b. Ireland, aged 50, married
Leech, William, b. Canada, aged 20
Leech, Jane, b. Canada, aged 18
Leech, Helen, b. Canada, aged 16
Leech, Samuel, b. Canada, aged 14
Leech, Sarah, b. Canada, aged 14
Kidd, Thomas, b. Canada, aged 8
This suggests that Samuel Sr. is James' father (living next door to each other, and James names his first son after his father).