Have you looked in Liverpool for the 1851 census?

Sarah Maria was baptized at St Peter, Liverpool.
She would appear to have been just 19 at her marriage in 1849.
If Mary Jane was born around census time, it would not be unusal for Sarah to be home for the birth and hence in Liverpool.
Birth of Mary Jane should then appear in the UK records.
As a mining family they could certainly have emigrated, though it would be after August 1864 when Charles Wesley was born in Ramsey.