My granddaughters didn't argue with each other, only with Mommy and I.
Maybe keep a balloon in your pocket and when they start yelling at each other
calmly start blowing up the balloon. And tie it shut.
Then step between them and pop the balloon. The noise should startle them
into stopping the fight. Tell them to take it outside.
I expect that after you do that a couple of times, they'll start to notice when
you begin to blow up the balloon.
