That’s exhausting. If you get together with them again, I’d start out by saying that you love to see them, but arguing is not fun, and you don’t want to see it for this get-together. Say it nicely, looking the kids right in the eye with a warm smile. Then if something starts, immediately intervene and say again that it isn’t fun to watch and ask them to separate. This is modeling for the parents. You can ask what happens at school when kids argue, to get a sense of what they act like there.
If the parents remark on this to you, feel free to say that it much be tiring as parents and ask if they’ve looked for any parent coaching classes.
If you know that something is going to be a trigger, like who gets to do something first, say before that arises that you want to try rock-paper-scissors first.