Co-regulation is about helping your child learn how to self-soothe when feeling overwhelmed. Children aren't born with this skill, so it's up to parents and caregivers to teach them how to manage their emotions.
When your child is upset, it's important to remember that they're in their right brain, which is all about emotions and creativity, rather than their left brain, which is all about logic and reasoning.
If your child is upset that they can't have their friend's toy, speaking logically to their left brain would sound like:
"You can't have that; your brother is playing with it."
You can see how this might make them reactive and start a tantrum. Instead, try speaking to their right emotional brain and, in a soothing calm tone, say something like:
"I know it's tough when you want something someone else is playing with. It's okay to feel upset about that."
Once your child feels validated, heard and understood, you can then start to connect with their left brain by suggesting a solution by saying something like:
"Let's go and play with the truck or bike while you wait your turn." or "How about we play a different game together?"
Using a combination of emotional right-brain and logical left-brain communication can help your child feel supported and understood while also helping them learn how to manage their emotions.
So the next time your child is upset, try co-regulating to help them self-soothe.