Make things safe immediately. Take away the toys being thrown, or get the child's hand to release her sister's hair.
Don't blame, shame, or punish. These actions further frighten children, and further isolate them. They add to the load of hurt that makes children aggressive.
Decide to whom you are going to listen first. Both the aggressor and the victim need your help. If you always spend your warm attention on the victim, the aggressor's problems don't get addressed, so it might make sense to decide to go to the aggressor as often as you go to the victim. Of course, the victim needs someone to check the damage done, and to offer warmth and caring. If it's the aggressor you are going to put first, you can tell the child who was hurt, "I'm sorry, love. I know that hurt. I'm going to spend a minute here with you, and that won't be enough, but I need to see Molly and help her--she must be pretty upset to do this to you. I'll be back." You also can keep the crying child close to you while you attend to the aggressor child, although it's harder to keep thinking straight.
Remember that children who hurt others didn't want to do it. They feel guilty and even more separate than before. Guilt erases people's ability to look like they care--no one looks more impassive than a child who has just hurt someone. The "I don't care" look is actually a cover--underneath, the child is heartbroken that she was left so alone and got so desperate.
Make generous contact. It helps children's guilt lift if you apologize for not having kept things safe. You can say, "I'm sorry I didn't see that you were upset with Ginger. It's my job to make sure things are safe. I know you didn't want to hurt her." It also helps to let the child know that the child she hurt will be OK. "Ginger is crying hard--her head hurts--but she'll be OK."
If your child can cry or tantrum at this point, healing has begun. Listen. Sometimes, your presence breaks the crust of isolation and the bad feelings can release. The feelings that pour out are the root cause of the problem, and your child is unburdening herself, with your help. Let her feel intensely for as long as it takes. She'll decide when she's done enough.
Often, a child who has hurt someone can't feel anything. The feelings of guilt button a child up tight. She doesn't feel safe at all. Your best course of action is to make contact with her by spending some moments--perhaps five or ten--paying attention and doing what she wants to do. This isn't rewarding your child for "bad" behavior. It's "thawing her out," helping your child recover her sense that you care. Without that sense, she can't function reasonably. But playtime with her won't heal the wounds she carries that drive her nuts. You'll need to wait for a little upset she brings up, like not being able to find her favorite toy, or you cutting her toast into squares instead of triangles. This little upset provides a back door into the tensions that have been dogging her. Listen. This is the crying she couldn't do earlier, when she was too afraid she was "bad."
Encourage her to come to you when she's upset. Children don't do this easily when they carry a big knot of tension, but offering the idea that you want her to ask for help indicates the direction things will go in over time, after many cries have released some of her fears.
Spend playtime with her and elicit laughter when you can. Connecting with a warm adult in play can be a powerful means of keeping her sense of closeness alive. It's that sense that will keep her on a good track with her friends and siblings.
