No offside and one player just stands by our goal

I would not have one child stand with the attacker all game. There is no offside in 5v5, so the forward is allowed to stay high. The useful question for your players is how to protect the goal without letting that one player drag everybody away from the rest of the game. I’d give a simple reference: the deepest player can keep the attacker and the goal in view, but they still need to stay connected enough to help when the ball is elsewhere. If another teammate becomes deeper naturally, the first player can move again. It does not have to become man-marking. You can recreate it in a small game with one attacker allowed to stay high and let the defenders work out when they need to cover the space. Treat it as a football problem, not bad behaviour from the other team.