Isaac Harvey was actually born in 1839.
GRO Ref : Q4 1839 - Okehampton - Vol 10 - p 165

Mary Harvey was born in 1841.
GRO Ref : Q4 1841 - Okehampton - Vol 10 - p 171

Mary was baptised the same day as Isaac was.

Charles Harvey is probably the son of John Harvey and Mary Balkwill married in Shebbear in 1809.

Charles Harvey was baptised in 1810 in Shebbear.

The reason I suggest this as the most likely is because there is an Isaac Harvey baptised to John and Mary in 1818 in Shebbear. Making him Charles's younger brother.

Also Shebbear is not that far from Hatherleigh and Northlew.

Images available on FMP.