What is it called when two people refuse to talk to each other?

What is it called when two people (usually kids) stop talking for a while after a fight or an argument because they are mad at each other?

As zondo says, above, such people aren't speaking to each other. (This is pretty much true even if they do literally say a few words for logistical reasons -- "I need to get out. Move your car" -- as long as they are doing what they can to avoid speaking.)

People aren't on speaking terms if they are doing this as a sort of policy long-term. You wouldn't say that if they're just too angry to speak for a couple of days, but you would if months go by and they are deliberately avoiding speaking.

The silent treatment is a deliberate plan to punish the other party by being silent, as opposed to avoiding speaking for some other reason (like being too angry or hurt to converse).

Such people may also be avoiding each other, but that doesn't imply anger.

This sounds like a classic case of the silent treatment.

Silent treatment (often referred to as the silent treatment) is verbal silence imposed onto another. It may range from just sulking to malevolent abusive controlling behaviour. It may be a passive-aggressive form of emotional abuse in which displeasure, disapproval and contempt is exhibited through nonverbal gestures while maintaining verbal silence. Clinical psychologist Harriet Braiker identifies it as a form of manipulative punishment.

Wikipedia

This could be used to refer to either one person not talking to another, or with two people they'd be giving each other the silent treatment, but zondo's comment of "not on speaking terms" is also valid.