What can be done about a mother who won't use a carseat?

I would report it to the local police department. They will be able to give you advice as to how to approach the subject to her, and even may step in themselves to enforce the law. It's amazing how irresponsible people can be, and even more amazing that these people decide to procreate. She can get into a great deal of trouble if she is caught, but if it is brought up to the police department, she probably won't get into as much trouble, they'll probably warn her. I'd continue to contact them every time she refuses to use a seat for her child. Hopefully the message will get across to her. *smh*