I had to do that very thing with my kids riding in the front seat of his truck. When I voiced concern, he FREAKED at the insinuation that he was not a perfect parent. More justifications that they "have done it hundreds of times before," or that "it isn't illegal." No, dumbass, but there are warnings all through the car that they aren't tall enough/old enough/heavy enough. I finally just got on the kids' cases about it.
Guess what? He was in an accident on the highway with D9 in the car today (his tailgating nonsense). She was a good girl--buckled up in the back, because she ASKED to sit there.