Powerplay is creating some odd games

I would mostly treat it as a new problem for the players. The extra player is supposed to help a one-sided game become more competitive, so it is not surprising that the picture changes quickly. I would not respond with a complicated powerplay system for U9s. You can give one simple reference: if they now have an extra opponent, can we use the ball and the space better rather than all chasing it? For the team with the extra player, can they recognise where the spare player is? Then let them experience it. I’d also resist coaching the original stronger team as if losing control of the game is a failure. The challenge has deliberately changed. Powerplay is a matchday balancing tool. It can also become a useful lesson in solving a different numbers problem.