mai mach , 60 , was stabbed 23 times with garden shears in the backyard of her albanvale home in western melbourne last tuesday .
hours earlier her grandson , alistair , 4 , was stabbed 18 times as he slept in his bed .
chinese tourist cai xia liao , 45 , has been charged with their murder .
parents will direct the remainder of the funds to a good cause .