They feel that nothing they are doing is working. All of these reactions only add fuel to the fire. Instead of reacting to the fighting, parents can choose to be pro-active. They can stay out of the fights in a nonjudgemental way. Children need to be able to settle things for themselves.
Parents can teach negotiation skills later during a calm period.
This will help them learn win-win skills that will be there when they are needed now, and useful in the future. Another thing parents can do is show confidence that their children will work things out. You'll be surprised. Or, the parent can get down on the children's level and lovingly put a hand out. They will give you the toy.