Both central midfielders keep following the ball to the same side

At U13 9v9, I would be careful not to mandate fixed left or right responsibilities. Assigning static zones often kills the organic decision-making you want these players to develop. Instead, give them a reference point based on the ball. My rule of thumb is that the nearest midfielder pressures the ball, while the second midfielder holds a central position to screen the space between the ball and the goal. They act as a pair that moves in a shifting block. If they both chase, they have lost the game of chess. Try a practice session using a 6v6 or 7v7 directional game with a wide zone on each flank where players cannot enter. This forces the midfielders to stay central and defend the middle space by shifting together as a pair rather than drifting to the touchline. Look for evidence in your matches where they communicate to decide who triggers the pressure and who provides the balance. That partnership is a key piece of their tactical evolution before they reach the 11v11 pitch.