As a dad with young kids, I can assure you that if this were to happen to them, I'd wage all-out cyberwar, on the kids and any parents that didn't properly curtail their child's behavior after official notifications. From doxing and redialing faxes to bully-vision youtube channels naming and shaming their family. It isn't slander if it's true…
I've made it 100% clear to my kids that I've got their backs. It's my duty as a dad.
Edit: Yes, this story obviously struck a nerve for me. I was bullied as a 12yo kid for a few months. I was a good kid, and did well in school, which made me the outlier.
I remember how terrified I was when the bell rang at the end of the school day, knowing that the pests would be waiting for me on the way home.
I also vividly remember the day my dad took me aside and simply said: "Go ahead. You have my permission to make them feel sorry. I have your back. Just don't do it on school property."
The day I took action was a truly glorious and empowering day. Hearing my dad tell their parents afterwards they had it coming, supported by some photographic proof of damage caused in the past, was even better. Upside: nobody ever bothered me again.
This happened a long time ago, and not in the litigious USA where money and lawsuits get off every bullying 'precious snowflake', yet I'm certain I'd take that risk today, if official channels failed to ensure the (mental) well-being of my children.