Oooo, no, don't send your kids to their dads loaded with leaflets, it'll cause Word War 3!
Have your kids complained themselves of the smell? Or is it that you dislike the smell on their clothing. In all fairness I really don't think you should be preaching to their father and his partner, I think they are at least being considerate and smoking on the doorstep. It's just inevitable I'm afraid that their own clothing stinks and it's being passed on to your childrens clothing.
If this were me, I'd say nothing at all and just wash their clothing as soon as they came home.