We did this last year at a resturaunt and I don't remember DS asking why Santa was not giving the little kids a gift. I think we just told him there are kids out there that were not as lucky as him and his brother and he wanted to help. We actually left, went to Target and came back with a toy. It melted my heart that he wanted to help!
I am one that will almost always donate when the cashier at the grocery store asks me, even if I am shopping with my calculator, I know it could be so much worse.