But even if she wasn't doing anything when the police got there, if the decision was made that the girl was going to be criminally charged, then arresting her is the natural next step in the sequence of events. I don't know who made the decision to press charges in this case, but the police were doing the job they were called to do.
There are just too many unanswered questions for me to have a strong opinion about this. It seems harsh but it also seems everyone was backed into a corner. And just where the hell were the parents?