Do your children fight (over legos, number of chocolate chips, parental adoration) for what is 'fair'? Does your singleton compare her/himself to friends, finding injustices in bed times, household rules or screen time?
First, you need to know that this is perfectly normal. And, it's good. Fairness is one of those moral categories that needs a lifetime of navigation and exploration. It can be really frustrating to feel like your kid is overly focused on it, but ultimately all of the 'little' issues (like a fair number of treats) are practice for the 'bigger', real-world issues. Use the small examples to practice for the big ones.
So, it's important to take fairness seriously. This article in Psychology Today suggests teaching about it by starting with those everyday examples. You frame conflict over fairness as an opportunity to practice empathy, collaboration, and creative problem-solving. You facilitate the situation and don't necessarily solve it. How?
This is where improv can enter. Say 'yes' to the situation, accept it for what it is by listening and really seeing what is going on (rather than jumping in to take over or fix it immediately). Often you'll observe the emotions hiding underneath the huffy It's not FAIR! That's a great first step. Listening. Observing. Stating what you observe and hear promotes empathy and understanding. It's as simple as saying: "You're frustrated."
From there, you have lots of options. Again, you tap into improv skills by thinking creatively and flexibly on your feet. Break out of well-worn scripts. Maybe you decide to model collaboration: you pretend to be one of the kids and offer an olive branch by asking what might feel more fair. Or maybe you encourage collaboration, give your kids a common platform to work together toward -- you take the treat back until they come up with a workable solution -- making it clear that it is not a punishment, that you aren't angry, but that you want them to work together. Or maybe you do a silly dance, getting them both to laugh and break the tension. There truly are endless ways to address fairness issues. Improv can open up the possibilities, and your creativity and spontaneity will model important life lessons along the way.