Ultimately, the best cure for a crisis is prevention. Watch the signs that your kids are getting frustrated with things, do a HALT check (hungry, angry, lonely, tired), and try to solve problems before they reach the fight-or-flight stage. But if they do reach that point - and it happens sometimes, especially with kids who are highly sensitive or have difficulty communicating their needs - these steps should help.
(The first bit is in response to a question about where to draw the line in terms of kids who are trying to fight one another.)
A great thing that Sandra Dodd always says is that the other child, the victim, has a right to feel safe in their own home. The aggressive child's "right" to be aggressive does not outweigh the other child's right to safety and peace. (The same applies to other adults in the house, of course.) So that is where the line should be drawn, I think, with regards to how much you tolerate.
However, that's about the expression of aggression, and I think it's important not to confuse the expression with the emotion behind it. You can communicate to the child that specific actions are not acceptable (a simple "Don't hit me/her/him!" is what I'd use - as opposed to "We don't hit" or "That was mean!" or things like that) while still validating their feelings. Usually a person who is behaving aggressively is flooded with adrenaline and cannot think clearly in that moment. They cannot think ahead to what consequences their actions will have. What we learn as we get older is how to slow that adrenaline flow, to breathe and calm ourselves down before we react. That's really hard for a kid!
When I was younger, I had a very bad temper, and often felt like I was going insane when I got angry. If I could have written a set of guidelines for people to follow when my anger reached "crisis" levels, it would have looked like this:
1. Don't ask me questions or talk too much. I am already overstimulated and more input will only make it worse, and in this state I cannot organize my thoughts into words.
2. Do not touch me, or any object I was just holding or using, unless you must do so to protect me or someone else. I may want a hug when I calm down, but if you come too close when I am raging, I will perceive it as a threat. My instinct will be to get you away from me in whatever way is necessary, no matter how much I love you.
3. Do not, under any circumstances, tell me that the reason I am angry is not a good enough reason to be angry. Don't tell me, in the heat of my anger, that I am being irrational, even if I am. Invalidating my feelings will only make me much, much angrier.
4. My adrenaline rises with the volume of your voice.
5. Make it clear to me that you still love me. I am just as frightened and unhappy about what is going on as you are, and I'm terrified that I will not be forgiven. If you make me feel like a terrible person for being angry, or you threaten me in some way, that only adds to my fear. Fear compounds anger.
6. Now is not the time for problem-solving. The thing that made me angry is no longer the problem. The anger is the problem. Once I have calmed down, we can solve the original problem, but not before.
7. I need to get this anger out. I simply cannot sit and stew in it. I need to yell, I may need to stomp, and I may need to physically destroy something (giving me some old magazines to rip is a good idea). If there is another person in the house who cannot handle my reaction then we need to be in separate rooms until I have calmed down.
8. Since I cannot think clearly in this moment, I need you to protect me from doing things I will regret. Don't allow me to injure people or pets, destroy anything that has value to me or others, or say things that will not be forgiven. Again, you may need to separate me from others and simply be alone with me until I am calm.
9. If I want to be alone, respect that. But do not force me to be alone against my will. I am frightened of my own anger and will become much more frightened if I feel caged and abandoned.
10. When I finally calm down (which happens remarkably quickly when all escalating stimuli are removed), I will be feeling trauma from the emotional stress we all just went through, fear that I may have made everyone hate me, shame and frustration with myself for losing control, and physical exhaustion from the adrenaline storm. *This* is the time when I need a hug - a long, close, "everything is okay, just breathe with me" hug. I will probably be sobbing at this point. Know that you are not rewarding my actions by comforting me - I am deeply remorseful at this point. You are giving me the security of knowing I can trust you in my very worst moments, and that is something I desperately need.
11. After I have stopped crying, washed my face, had a cool drink, and we've all laughed together, I'll be able to handle some gentle problem solving. After we've all been emotionally "emptied out", so to speak, it will be easy to talk through what happened and figure out what we could do differently next time. After what we've just been through, we'll all be eager to remember a better solution so it doesn't get this bad again.
12. At this point, one of two things should happen: Either I or the other people involved will be exhausted and want to be alone, or we will be eager to bond again to prove to each other that everything's okay. This is a good time for a quiet board game or a lighthearted movie together, some cuddle time, and some ice cream.