How do you stop confident U7s taking over every decision?

I’d keep the leadership but put boundaries around what they are allowed to lead. You can say, “I love that you are helping. The coach chooses the teams and everybody gets a turn on restarts. You can help by encouraging your teammates.” That gives their confidence somewhere useful to go. I would also deliberately create moments where the quieter child has the responsibility: they start this game, they choose the bib, they take the next restart. Do it without making a big announcement that you are protecting them. Strong personalities are not a problem. They can become brilliant teammates. They just need to learn that leadership is not the same as making every decision for everybody else. That is a useful social lesson at six as well as a football one.