Hi Ruth. This child should have his/her own therapist, in addition to a therapeutic environment. His/her providers or parents should share with you how to take care of him. What is the child's treatment plan? What medications is he/she on? An adjustment to medication might be a first step to reduce frequency of outbursts. You can help by keeping a "diary" of outbursts, frequency, and triggers. That will help everyone see any patterns or common triggers.
My daughter with high ACES score became very violent when she hit her tween years. Her therapist used heart rate variability (HRV) training with her to reduce her anger and learn to regulate (you can look for an app called HeartMath), and then worked with a neurotherapist to do neurotherapy to reduce activity in the brain's amygdala (fight/flight center). It takes time, and she continues on medication, but she can now go through daily activities without punching a wall or a person.
Finally, as far as your immediate question as to how to handle this child in the heat of the moment, using your words to help him/her express rage might diffuse a bit. You don't have to get him/her to talk, but you can express for the child: "I see that you're very upset. When you saw that there were not computers available, you felt very angry (or left out, or frustrated...)." Our kids don't now how else to express their rage (often legitimate, given their pasts), and by validating the anger you can give him/her tools to use feeling words.