That's a huge question.

I guess the main thing I would say is. You can help. But you can't fix everything. I'd tackle the areas that you have most control over. The two children who visit, I don't think there's much you can do there. Other than consider reporting them to Child Services. Given that a lack of potty training. Could indicate other kinds of neglect too. And even if there isn't, it might be that professional involvement. Opens up a door for getting additional support.
In terms of helping the six children you are mainly working with. What support does the family have already? Is there a psychologist or family support worker involved? I wonder about using something like
ABLLS to assess where they are. As that can help target incremental skill acquisition. I've used this, but only for one child. However, you could perhaps focus on a different area with each child. Rather than trying to cover everything. It doesn't tell you how to teach / support skill development. But in working with a single child. I found it very helpful to break down the steps for each skill. And then it was easier to devise activities which exercised those.
As a general idea for the older children's interactions. I wonder about playing a short game together each day. Board games, as long as they don't take too long. Would help to support turn taking, rules following and listening.
Also, making sure that they are all supported in developing a unique hobby or special interest. That helps them have something that's "theirs" to focus on. Whatever they choose will teach certain skills over time. And maybe it'll give them something positive to talk about. Either between themselves. Or with adults. So they're getting lots of positive attention. Rather than having to act out to get noticed at all.