depends what type of people they are, if they are nice then i would invite them over for dinner ans mention it then, kill them with kindness.

but if they are not decent, then i would try to be nice and say something like " wow, your gracie is a really pretty dog, you must be very proud. you know i noticed that she seems to be going to the bathroom in my yard and was wondering if you could maybe keep an eye on her while she is outside?"

then mention the traffic and the fact that their gracie may one day become a statistic.