Each child gets a clearly marked territory on the floor. Two areas separated by rope, a row of books, or tape. Each builds or creates their own world inside their space. Entry to the other's kingdom requires a knock and permission. The physical border is the mechanism: it gives each child something to govern, which replaces the conflict motive (fighting over the same space) with a governance motive (ruling your own). A child who rules a kingdom has no need to invade one.

What you need

Rope, tape, or a row of books as a visible border
Each child's own toys — not shared

How to do it

1Mark two territories on the floor — rope, tape, or a row of books. Make the border real.
2Each of you has your own kingdom. You're in charge of everything inside your border.
3To enter the other's, you knock and wait.
4Leave them to govern. A child who rules a kingdom has no need to invade one.

Variations

If they won't stay on their side: shrink your own kingdom in front of them and keep playing happily inside it — a smaller, clearly-enjoyed territory sells the idea better than a rule does.
How it ends: announce a "kingdom tour" where each ruler gives the other a guided walk through, then pack up together.
This one builds cooperation before it's needed. For an active fight, use The Separate Reset (B006-20) instead.

💡 Parent tip

Enforce the knock rule once, seriously. After that, the children will enforce it on each other. The border only works if it's real. Treat it as a genuine property boundary, not a game element.

What your child develops

Territorial respect — a physical border teaches boundary negotiation without requiring parental enforcement of sharing rules
Independent world-building — each child creates without competing for the same creative space
Autonomy and ownership — governing a space builds self-determination that reduces the frustration driving conflict