Hi Lili,
Unfortunately none of these children will be in Denny Cemetery as it wasn't established until early in the 1880's.
I have MI's for the other churchyards in Denny, the Old Parish and Broompark (existing stones only, not burial records) and there's no sign of them there either.
According to Dennyloanhead Church's Burial Records, there's a Margaret Kennedy (wife of James Risk) who died 28.06.1860 age 29 buried in Dennyloanhead Churchyard but there's no stone. There is however, a big family stone to the Risk family in Denny Cemetery which lists Margaret Kennedy.
The inscription is as follows:
"Erected by a few friends in affectionate remembrance of ELIZABETH STARK
Second daughter of the Rev. James Stark, D.D., Dennyloanhead
Who died 25th September 1887 aged 84 years
And of MARGARET MONCRIEFF STARK his grand daughter
Who died 24th January 1934 aged 82 years
CATHERINE ARCHIBALD RISK died 7th May 1881 aged 16 years
JANE KNOX RISK died 2nd April 1882 aged 16 years
JAMES CHARLES RISK died 3rd February 1888 aged 11 months
Interred at Dennyloanhead
In memory of MARGARET KENNEDY wife of James Risk
Died 28th June 1860 aged 29 years interred at Dennyloanhead
JAMES RISK died 24th May 1898 aged 76 years
JEANIE KNOX wife of James Risk who died at Edinburgh on 1st June 1922 aged 87 years
JAMES RISK youngest son of James Risk died 24th January 1961 aged 86 years
Cremated in Edinburgh
MARGARET RISK eldest daughter of James Risk
Who died at Stirling on 21st December 1936 aged 81 years
MARY RISK daughter of James Risk who died at Stirling on 22nd November 1950 aged 91 years
The Burial Ground of JAMES RISK of Bankier"
Do you have the family's whereabouts on the 1841 Cencus? If it was a case they lived in Falkirk, then the chances are they may be buried in one of those Churchyards which have mostly all been cleared - either for landscaping or the land used for re-development.
I had a look through the published pre-1855 MI Books for Stirlingshire and there's no Kennedys listed in any of the Churchyards there either.
Sorry for all the negatives!
Anne