I have two kids and one thing is guaranteed, if they are off playing by themselves its just as matter of time before they are fighting. I usually let them work it out, but when it escalates to a certain point they usually come screaming for me or my wife. Of course, not being in the room where it happened means it’s really hard to decipher what happened and everyone trying to speak at once is impossible.
In these situations what I do is look at one and say tell me what happened. As their story begins the other will always try and interrupt and its important to shut down the interruptions. Then after they are done the other gets a turn in the same fashion. Their stories always remind me of this Proverb
The one who states his case first seems right, until the other comes and examines him. Proverbs 18:17
Both stories seem to allows diverge and it’s hard to figure exactly what happened. Usually by the time this routine is over both have calmed down and go back to acting reasonable.