The number two way to discipline a child under the age of 7 (and older!) is by having the child make restitution. Not when everyone is upset, not when everyone is crying, not when everyone is angry. That is when you need CONNECTION first (see this post regarding connection: https://theparentingpassageway.com/2010/02/22/the-number-one-way-to-discipline-a-child/ ) But, after, everyone has calmed down, then you sit down and take up a paper and crayons and draw. And when the child comes to see what you are doing you can quietly say (but NO GUILT! NO LECTURES! NO BOOK ABOUT THE INCIDENT!) “I am drawing a picture for your sister because she was very sad earlier.” Or quietly start to fix that toy and when the child comes you say, “Could you help me glue this part? I am fixing this for your brother.”
I think this technique would work well for children that are about four or four and a half up. For children younger than that, really they need the connection part most and as they grow older they will learn about restitution. Of course, they can help hold an ice pack or watch you fix something though.
The point is, though, first SPACE….everyone needs to calm down. I am against time-out for children, unless you want to take a time-out for yourself. Nothing can be accomplished when everyone is yelling and screaming and you must be calm in order to guide and to teach. Then, connection. Take that sobbing child on your lap, hold that child. The boundary is NOT changing, the boundary is still there, but the connection is there. The child is adapting to the boundary. (If the boundary keeps adapting to them, they are learning nothing). Then later, when things are better, make the restitution.
Those are the keys you need for success and for guiding. Our goal is to raise wonderful adults, not to punish a small child over an incident they will not remember years from now. But over time, it will become engrained in them to approach conflict with a means to provide space, to connect and to problem-solve.