alistair mach , four , and his grandmother mai mach , 60 , were murdered .
chinese tourist cai xia liao , 45 , is charged with their murder .
fundraising page for victims ' family has raised almost $ 50,000 in one week .
the funds will help pay for their funeral and be directed to a ` good cause '
liao had allegedly been having an affair with boy 's grandfather , brian mach .
police allege liao stabbed alistair in his sleep while mr mach was gagged .