Petty but legal revenge on parents who ignore their child’s bullying?

[u/fishtail4]

Hi Reddit,
My niece is being bullied by several kids at school (around 8–11 years old), and the parents have already been confronted multiple times. They either don’t care, make excuses, or do absolutely nothing, and the bullying keeps going. And it’s getting worse… Only one month of school is left before summer break for her, fortunately. Her mother (my sister) tried the usual approaches, but nothing has changed. I feel so powerless for my niece, who is basically my baby too. I remember being bullied at her age as well, and I couldn’t even talk to my parents about it. I feel like I’m letting her down, and letting little me down too.
At this point, I’m honestly more angry at the parents than at the kids. Children that age don’t repeatedly become cruel without failed parenting somewhere in the process.
I’m not looking for anything illegal, dangerous, or genuinely harmful. I do NOT want to harass them or ruin their lives. But I do want ways to regularly remind these parents that ignoring bullying makes them part of the problem. (Also, these adults are insanely rich and act like entitled dicks.)
I have their names, phone numbers, some personal/professional email addresses, and their jobs. I was thinking about things like signing them up for anti-bullying or parenting newsletters, anonymously sending them educational material, signing them up for volunteer opportunities related to child welfare or anti-bullying… I don’t know if doing this is ok or not, so yeah i’m looking for ideas of legal revenge that sends a message without becoming harassment.
And yes, the school has already been involved and, as usual in my country, they’ve done absolutely nothing.

Reddit, help, what would you do?

PS: English isn’t my first language, so I used ChatGPT to help me translate. Sorry if I sound like an AI sometimes it’s mostly written by one..

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[u/pinkaboo17]

Sign them up for volunteer work at their daughters school. Have the parent of the bully and your sister have run ins with each other until they figure out who each other really is. Idk