As a childminder registered to have three children under the age of 5, I can only earn three times my hourly rate at most. I do feel guilty about not offering a reduction for twins/siblings (which I did offer when I was new to childminding but was taken for a right mug), but I need to earn money as much as the parents paying me need to earn money.
As a parent of twins myself, I totally understand about childcare costs. We are currently putting ours through university at the same time.
I know many childminders who do offer reductions, so they are out there.