The police are not going to be overjoyed at hauling a 6 year old down to Juvenile Hall for acting like a 6 year old. The girl's parents are hotheads.
It could be that your grandson has some behavioral issues which could be improved if dealt with now. Maybe he doesn't and the girl is being untruthful. His parents should consider having a talk with the teacher and maybe a school psychologist or behavioral specialist to see if his conduct is a concern to them, and if so, come to some plan about how to address it.