I have not asked this of parents before, but I have heard horror stories from other caregivers who have children coming to daycare reeking of cigarette smoke. This would bother me a lot. I think you have a right to ask whatever you want...it is your home and your business. I would just mention that you have a sensitivity to scents including cigarette smoke and that you unfortunately cannot accept children from smoking parents for this reason. Many parents ask the same of us.