One way would be to check the workhouse records - note that freebmd is not complete yet and some birth records for 1840/1841 are currently missing, so that might be where they're hiding. Or Charlotte was a middle name, maybe? (I presume the workhouse census was for 1851, not 1861? Any other Smiths in the workhouse in 1851? I would expect their mother would also be in unless she had passed away, but as families were often seperated she might be on a seperate page.
Also if they were illegitimate (one of my ancestors, a young widow, had two illegitimate children by the same man, spaced three years apart), they might be listed under a different surname. Was Catherine also b. Ponsanooth?
There are no Smiths in Ponsanooth in the 1841 census (it is a village in St. Gluvias), but in 1851 there are:
Jane Smith, visitor in the house of Edward Martin, a gunpowder maker, aged 37, unmarried, and
Emily Jane Smith, visitor, aged 1, presumably her illegitimate daughter. (they were b. Cambourne)
also
Mary Smith, lodger, 26, unmarried, in the house of Walter Hill, 76, and Jane Hill, 74. (she was b. Wendron)