Do you coach using the weaker foot at U7 or just let them play?

I would give them reasons to use both feet rather than making the weaker foot a rule. At U7, I like little pictures where the easiest solution happens to be on the other side. A gate on the left, a defender approaching from the right, or a 1v1 where they need to move the ball away from pressure can create that naturally. You can also praise the attempt when they use the less comfortable foot, even if the touch is messy. What I would avoid is “left foot only” for long periods. That can make a player ignore the best football decision just to satisfy the condition. The goal is not two identical feet at six. It is a child who gradually becomes comfortable enough to use whichever foot helps in that moment.