No, I suppose the police couldn't prove anything unless they pulled that car over and caught her doing it themselves. But it's like reporting dangerous driving, which I've done a few times; even if the person is driving beautifully when the police catch up with them, it still scares hell out of them being given a warning. Plus the fact that they were warned goes on their record, so if they're caught again, the penalty is more severe. (I think. Please correct me if I'm wrong.)
Anyway, today's Herald Sun has an article on kids being improperly restrained in cars. I can't post a link here, but it's clearly visible on their website. Really, I'm surprised anyone would ever knowingly risk their child's life.
Kirsten