One for all and all for one
When I have to punish my kids, I punish them together. I want them on a team against me because I can always forgive, always make them laugh again, and repair any rupture. They might not be so skilled at fixing their relationship, so I’ll keep that strong for them. They’re a team, working against me.
It works like this. My son, D, calls out in pain, because his sister, H, just snatched a toy off him. He comes running to me to complain about it. My summary judgement? H, never snatch and D, stop telling tales. Play nicely, or you’ll both go on the naughty step. Or: H is screaming because she wants to play Minecraft now, and D is ignoring her. Result: H, stop screaming, D stop ignoring your sister, or neither of you will get the game.
When something goes off the rails, and one of them is screaming or snatching or slapping, it’s almost always provoked somehow. One didn’t suddenly walk up and start fighting, the other always did something to kick it off, causing a spiral of problems. I want to make it clear that the most important thing is that they get along. I don’t care who plays Minecraft or who has the toy, so long as they can resolve their differences. If they don’t, they both suffer for it.

In our house, toys don’t belong to either child. Everything is for everyone’s use, like a lending library. If D had a toy that H wanted, then she couldn’t have it straight away, but it was her turn in five minutes. Of course, every toy had a nominal owner, that the other generally left alone, but they all had to be shared. That taught patience and to appreciate their time with the toy. It also avoided us paying for two of everything, when in practice they would both want it for the same five minutes then it would sit in a cupboard for the next two years.
Punishing both kids together is also a useful default in the common case of having insufficient information. Picture the scene: you are busy washing up when the screams start from the living room. One child claims they were punched, the other that they never did, that they were wronged by having their toy stolen. Both look up at you with righteous indignation in their eyes, awaiting your judgement. Who knows what happened? Both will stick to their stories now, and the truth is probably a combination of both tales. “Play nicely, or you both go on the naughty step,” is a handy default reply. The problem is that they were unable to resolve their differences for themselves, and for that, they are both punished.
Having given an ultimatum to them both, stick to it. If they don’t take the hint and you have to punish them together, remember to forgive them quickly. Once they’re back and happy, then reward them with a good bout of tickling or their favourite game.
Your kids’ suffering and joy are linked. If they can’t get on, then they are both punished. If they can play and have fun together, then that is its own reward.