I see a death for Susan with dates to fit your Robert
Digger - Great War Index. Victoria 1914-1920
Given Names: Susan
Mother: Unknown UNKNOWN
Death Place: Kaniva
Reg. Number: 2119
and I see the death in SA for Daniel...BUT considering they all died in different areas they are probably all buried in different cemeteries..
There are a few cemeteries online for both VIC and SA (maybe someone will search them).