I would not put them in fixed zones, but I would create a game where being on different heights gives them a clear advantage.
For example, use a central 4v4+3 where the three midfield players can move freely. If all three stand level, they will quickly find the forward passes blocked.
Ask them: “Can one help underneath, one between and one beyond?”
Those are relationships, not permanent roles.
Then let them rotate depending on where the ball and pressure move.
At U13, the players are ready to understand that a midfield triangle is not just the shape you draw before kick-off. It constantly stretches, flattens and reforms.
The aim is not to preserve a perfect triangle.
It is to make sure the players are rarely giving the opposition one straight line to defend.
