Yeah, it's called showing this to a police officer and filing a report.
You don't threaten to harm children.
Also, word of advice-- never fight outside a courtroom when kids are involved. Just take my word for it. Heck, I'm sure there are others on here who will second my advice on this one. You don't need to be talking to her. You don't have a relationship with her. Any problems she's doing, you can file reports. Maybe Dad will get a clue and put her on a short leash.