Alfred died first on 29 August 1848 and was buried on 31 August.
David died on 10 October 1848 and was buried on the same day.
David's death was registered at St James Church of England, Sydney, and Alfred's at St Lawrence Church of England, Sydney.
Where they were buried is unknown.
I have been told by a St James archivist that the church did not have a burial ground at that time.
Also that a funeral service was not held in a church in those days, the minister going to the grieving family's house to pray over the deceased instead.
The two boys were not buried in the Devonshire Street cemetery, according to a book that records all the burials there.
[Sydney Burial Ground 1819-1901 [Elizabeth and Devonshire Streets] and History of Sydney's Early Cemeteries from 1788 (2001) by Keith A Johnson and Malcolm R Sainty, published by the Library of Australian History, Sydney]
Nor are they buried at Balmain Cemetery with their older siblings, James and Grace.
I have concluded that two Mawbey boys must have been buried in the Devonshire Street Cemetery, and that their records have been lost.
They were buried in the same cemetery as the mysterious Sarah Barckley Mawbey who was buried in April 1848.