Children are everybody's business but we also need to understand that not everyone can be a perfect parent all the time. Any parent who never compromises on any aspect of their parenting either has the kind of support I can only dream about or they're lying.
I'm sure this woman knows that leaving her children in the car isn't ideal but she's not leaving them there in an alley while she turns tricks to buy crack or puts her wages into the pokies. She probably has reasons for doing what she's doing and without knowing those reasons it seems rather intrusive to report her to the committee. Children are run over in school car parks far more frequently than they are taken with stolen cars.
If you're concerned then a better way of expressing that concern is to be part of the solution - invite her to park her car near yours so you can watch her children too.