Regarding marriage between William Casson and Sarah Greenwood are following births.
Fanny Casson b1849 mmn Greenwood.
Elizabeth b1851 mmn Greenwood.
Hannah 1853
Death for a Sarah Casson aged 31 in 1858
An Elizabeth Casson born 1851 Manchester can be found with William Casson and wife Margaret in stockport
I found the births for Fanny, Elizabeth and Hannah Casson on the GRO index, all have mother's maiden surname Greenwood.
Fanny born 1849 M quarter Manchester, died 1849 J quarter Manchester age 0. If I have the right person. Might explain why she isn't on the 1861 census.
On the 1861 census Elizabeth is age 10, living with father William, (step?)mother Margaret and sibling Hannah, in Manchester/Stockport.
The only possible information I can find for this family after that anywhere is the 1870 United States Federal Census for Illinois has William Casson age 44, wife Margaret age 34, and daughters Elizabeth 19 and Alice 16. All born in England.
I couldn't find the marriage between William Casson and Margaret, to see if her age is approximately that shown on the 1870 US census.
That 1870 US Federal Census I found on familysearch.org, and looked at the original document, and wonder if the name Alice was mistranscribed as the handwriting is unclear. Could she be Hannah?
I could be totally wrong, but none of this family are on any English censuses after 1861, though I haven't found them on any passenger lists either. They would most likely have sailed from Liverpool to the USA?