Not buckling kids in car seat correctly 😓

So I have in laws that don’t take car seats seriously since 8 months their kid was front facing. Since one year they haven’t even bothered buckling him in correctly at all because he “doesn’t like it.” They don’t use the chest clip only the crotch buckle. In addition to that the harness tail is complete sucked into the seat (not even tightened slightly! He is 4 now - in my car I buckle him correctly and he fuses maybe a minute then he is fine. Safety is not optional. My concern also lies with my daughter as they took her places without a car seat. When I gave them car seat they don’t even hook it up properly. How to I address this without causing conflict? I’ve mentioned in the past gently a few times but nothing.