Kid assaulting my child at homeschool meet ups

Edit: to clarify.

The behavior has been ramping up It started with little things to tease her and has escalated to full out throwing something at her the last time we were there.

I have a younger son who has had some behavioural problems which has completely resolved. He was just turned 4 when he was struggling to integrate himself into society after COVID. So I was kind of trying to give other parents some grace, as they did for me. I didn't want to overreact for this reason. But not enough is enough.

This kid keeps throwing things at my daughter. He is 11 years old so there is zero excuse for this. I didn't make too much of a scene the first couple of times but I've literally had it now. They are hard objects like it could really hurt her. My daughter said she told the Mom and she just shrugged. I haven't been back because of this, but my kids also miss some of their friends.

What should I do? I am not interested in a hen fight with any Mom, so far we have remained totally neutral. But what can I do about this? The other Moms see it and some of them are very shocked and disappointed by it but the other Mom is never around, and they just get off scott free. This behaviour is unacceptable. If he was older he could be charged with assault. An 11 year old nows better and he doesn't have any sort of disability as far as I know either.