Hi, Grandparents don automatically have rights with regard to their grandchildren. That said children have a right to maintain contact with grandparents and it is well accepted that this is an important relationship in a child development.
The first step is to try to discuss the situation with the parents to arrange visitation. I'm assuming that you are asking because this isn't possible. I believe you can make an application to the court for parenting orders but this is probably something that should be done through a family law lawyer. They would be able to talk you through the process because you would need to undertake dispute resolution prior to the application to the court.