Before I call I think I might actually tell the mom the situation you are in. Yes she is going to know it is you and be really mad but if she gets caught then she also has no one to blame but herself because you warned her. If you say to her that because of your position you are obligated by law to report - and hand her a printed copy of the rule so she can see that you aren't just making it up and include the penalties she could be facing. Then if she still refuses then call. If she leaves your place with the kids buckled then well at least you have covered yourself. Of course the guilt of knowing she likely isn't doing them up on other trips is something that may make you want to call anyways. I am going to assume that it isn't that far from your house to theirs and that is why she thinks it is ok even though we know it isn't.