Openly, compassionately and with a firm tone. She MAY have smoked around them, even in the car. Isn't it cold where you are? You can mention you can't prove it, and you certainly are taking her for her "word", BUT I would mention you suspect it, and if that's the reason, then you are very disappointed in her, with a possible loss of trust.

With all you've been through, who needs a favor like that?
She needs to know what you've been through. You might even say if it WAS true, it was a very selfish thing of her to do, to put your children at risk.
I'd certainly do it. Some people often need to hear the truth, in order to understand it.

Before they banned cigarettes in the workplace, I was subjected to second hand smoke, and started developing symptoms of heart disease. They've all reversed themselves by now, but it was freaky. (I'm allergic to tobacco, as well, the highest of my allergans). Here in NC, I think the workplaces are smoke free, but some of the restaurants are not. I can't stand the smell of them and think anyone who still smokes has a death wish.