Self-control isn't a personality trait. It's a skill, and kids practice it most in the parts of play that look least like "learning." Dramatic pretend. Group games. Quiet boredom. Here's what's actually being trained, and how to set the table for more of it.
Self-control is built in the parts that look least serious
Pretend play is rehearsal for staying in role
When your child plays "mom" or "dog" or "astronaut," they have to act a certain way for the next ten minutes. The astronaut doesn't bark. The dog doesn't use the spaceship. Holding that line is hard, and that's the point. They're practicing "I want to do A, but I should do B," which is the core of self-control.
Especially impulsive kids tend to gain the most. Pretend play hands them a way to practice waiting and resisting that doesn't feel like waiting at all.
Group play teaches their nervous system to share the steering wheel
When kids play together, they have to read other kids, take turns, and adjust the plan. Each of those is an emotional regulation rep. The friend says "no, that's mine," and your child has to handle a small jolt of frustration without quitting the game. That's real practice. You can't simulate it with flashcards.