Pick a real decision the family has to make — the weekend plan, a meal, how to rearrange a room — and hold a short meeting about it.
Everyone gets an uninterrupted turn to say what they want and why. Write each position down where all can see.
Then decide together, and say out loud how the decision was reached. If a child's idea wins, it has to actually happen.
Real influence over a real outcome is what makes negotiation worth learning; a rigged meeting teaches the opposite lesson within one round. Writing positions down externalises the disagreement so it can be examined instead of defended.