Looks like you will have to go to Pietermaritzburg while you are here as I have found the following two estate files. I also found a copy of the marriage on familysearch.org. If you search under Africa and type in his name it pops up and you can download the scan. Once you have copies of the estate files from the archives it should give you more information on Edith and possible other children that they had.
Hope this all helps

Do enjoy your visit to South Africa!
DEPOT NAB
SOURCE MSCE
TYPE LEER
VOLUME_NO 00
SYSTEM 01
REFERENCE 1851/1945
PART 1
DESCRIPTION CALDER, ROBERT JACK. S/SP CALDER, MARY.
STARTING 19450000
ENDING 19460000
DEPOT NAB
SOURCE MSCE
TYPE LEER
VOLUME_NO 0
SYSTEM 01
REFERENCE 950/1965
PART 1
DESCRIPTION CALDER, MARY. BORN HENDERSON. BORN IN EDINBURGH, SCOTLAND. PRSP
CALDER, ROBERT JACK. DECEASED ESTATE NO. 1851/1945. DECEASED ESTATE.
STARTING 19650000
ENDING 19660000