I intervene whenever I hear anger and frustration. Actually, what I do is inquire "I hear angry voices. Do you need help with a problem?"
If at least one person has a problem, we go through our household problem-solving technique. All siblings are considered equals at our house, unless an older sibling has specifically been put in a position of authority.
Solving Problems Between Equals
1. Define the problem. Talk to each other until you agree upon the definition of the problem. Define the problem in terms that don't blame anyone.
2. Work together to think of ideas that might solve the problem.
3. Work together to pick a solution.
4. Try that solution. If it doesn't work, go back to step 1 and try again.
5. If this doesn't work, get help from an authority figure.
Solving Problems Between Equals with a mediator
1. Define the problem. The mediator helps each person take turns speaking. The mediator assists each person to define the problem in non-judgmental terms. The mediator helps summarize what each person thinks the problem is.
2. Work together to think of ideas. The mediator helps each person to be able to express their ideas. The mediator lists out ideas. The mediator may add in some new ideas as well.
3. Work together to pick a solution. The mediator does not pick a solution for the people having a problem. The mediator may help with practical information like “everyone won't get a turn if turns are an hour long.” Final decision as to acceptable solution needs to be from the people with the problem, however.
4. Try the solution. The mediator may leave at this point, or may stay involved to make sure the proposed solution is being implemented as agreed-upon.
5. If things aren't working, repeat the process. The mediator may also convert to the tools of authority and intervene with rules and instructions.
It takes quite a bit of training, but the kids do learn how to do this process on their own. Even 2 year olds. The kids also get highly motivated to resolve problems before they come to my attention, since stopping play to go through a mediated problem-solving session is *not* their idea of a good time! With young children, don't be surprised if the problem is "resolved" by one or both kids getting bored with the problem-solving process and wandering off to do something else.
Also, be aware of all the emotional vulnerabilities that come with a child in foster care. Some problems may be normal sibling stuff. Others may be more how a traumatized child expresses and process their pain. Telling the two apart can be very difficult.
Good luck and thank you for doing foster care.