So Alexander and Caroline had evidently migrated to Australia, married, had two daughters, returned to England, had a third daughter and crossed the Atlantic; Grace was their fourth daughter; then they travelled the relatively modest distance to Pennsylvania, where their first son and sixth daughter were born. It is easy to imagine Caroline as typically pregnant, at sea and constantly looking round in case small children disappeared overboard.
If they gave their ages correctly, Alexander must have been born between 17/9/1812 and 16/9/1813 and Caroline between 17/9/1817 and 16/9/1818.