I have a puzzle to unravel, and I could use a little help please.

I have John Stowell, (age 47 in 1851) married to Ann Killip (age 26 in 1851)
John was consistently listed as a miller in various documents, but it appears that he moved around a bit.
Their children are:
- John Stowell (born Sept. 6, 1846 – Lezayre)
- Ann Jane Stowell (born July 27, 1848 – Lezayre)
- James Stowell (born March 28, 1851- Lonan)
- Elizabeth Martha Stowell (bapt March 13, 1853 – Kirk Lonan)
- Catherine Margaret Stowell (bapt June 2, 1855 – Kirk Michael)

Catherine Margaret’s baptism record shows the following:
Child’s name: Catherine Margaret, Posthumus, daughter of
Parent’s names: John & Ann Stowell

I took "posthumus" to mean that she was baptized after she died, but now I’m wondering if I’ve interpreted this wrong. I’ve been trying to find this family in Brian’s 1871 census index, as well as on the 1871 census on Ancestry.com.
I can’t find my g-grandmother Ann Jane, or her parents in 1871.

In 1871, I did find an Elizabeth Stowell, age 18, born Lonan, working as a servant in the William Christian home in Lezayre. That could be my Elizabeth Martha Stowell.
In the household of Robert Kneen, in Balaugh, there is a James Stowell, age 20, born Kirk Lonan, working as a farm servant and a Catherine M. Stowell, age 15, born Kirk Michael, working as a servant. This information seems to fit with the information from their baptism records, BUT…. What about Catherine Margaret baptized posthumus?

Could someone with access to the 1861 census please check to see if this family can be found there? I’d especially like to know if Catherine Margaret was still alive on that census, and if she is ……… Are both parents still alive too? What are the parents ages in 1861? Does it give their place of birth?

Any assistance with this would be greatly appreciated.

Thanks.