H.C.
I tend to be blunt when it comes to things like this, so if it were me, I would have told her kids to get thier seat belts on. No one rides in my vehicle, no matter what age, without thier seatbelt. I have a 15 year old step son, whos mother never made him wear his seat belt since about age 5, but in my car, he strapped it on every time. It's a matter of knowing who's going to let them get away with it. My son is only two, but he knows that he can only ride in his car seat and we have to wait to start the car until he is buckled. I know you want to keep this person as a friend, so I would either approach it in a phone coversation as like, hey I noticed the other day that you and the kids did not wear your seat belts, can we talk about this. OR next time you guys go somewhere together, just hop in the driver's seat and say "is everyone buckled in so we can leave" and make the coversation go from there. Let her know that you won't transport them with out proper restraints. You just might change how she feels about it in her own car when she sees it's the right thing to do, and by the way - IT'S THE LAW!