OK op, experience here from three kids, last one of who is 9, ADHD & autism. Quirky personality shall we say. Also 20 years as HR Mgr handling loads of employee grievances & bullying.
What you want to achieve determines what route you take.
Want your boy to be able to handle these issues himself? Don't over manage it for him, focus your energy on him, building in him the skills and resilience he needs. You may be more upset than he is. Don't disable him. Harsh but true for us protective mums.
Want those kids taken to task by their parents? Guess what, you have no control over their parenting. What you would do in that situation ('I would want to know') doesn't matter... Because they will do what they like. Maybe that bad language or mean play is OK in their house.
If you confront the child or the parent in public to say 'your kid did this and I think you ought to know' you are asking them to immediately chose between your child and their own/their parenting. Hence, as with pp, you are v unlikely to get a good result. Automatic defensive position. You've come out fighting, they'll do the same. 'Who is this woman and kid saying me and my kid are crap?' They're bound to counter attack you regardless of the truth in the middle!
If you think these kids are mean to yours in school, and are likely to continue to be mean in school, there's your course of action. School has that lovely set of rules every parent has agreed their child will follow. A policy, school rules, house rules, class behaviour rewards whatever. So not following isn't just a reflection on their parenting, it's an objective - and agreed - standard. Helpful see? Less emotive, better result, plus an intermediary in school. Secondly, if it is than situation of one usually decent kid getting carried away by one strong willed bully, rather than a campaign of continual, planned meanness by the same group of kids, that will be known by the teacher.... Info you can't possibly know. More than one kid could be affected by this behaviour. Your complaint could be contextual intelligence eg added to multiple concerns about same child being unsupervised on zoom, using bad language, linked to references of accessing age inappropriate material, arriving at school unaccompanied....family needing support. Or, many concerns from different parents about increased online activity is a good time for teachers to at some small online group activities around staying safe and happy online at this time.
Right now you are rightly upset and angry and hurt. But take a moment to use that energy productively for your boy to achieve the best outcome.
Caveat: I'd want to rip their heads off. Then after I'd finished with the parents, I'd start on the kids. I am super Headmistress polite/serjeant major when riled. In a scary psychotic way. 'Now children. Play nicely in future purleease. I do not want to have the need to talk to you, or your mother or father about this again. Are we quite clear on that? 😲😉😁 Brooks no countenance (no idea what that means but likes sound of it)