Can you actually coach defending without making a defender?

Yes — coach the defending picture, not a permanent defender. At U7 I would use very simple ideas: “Can you get between the ball and our goal?” “Can somebody help the player going to the ball?” “Where is the danger if we all chase?” Those are defensive decisions any of the three can make. You can also praise a child for recovering and helping rather than only praising the tackle. What I would avoid is turning those principles into fixed starting positions. If one child happens to be deepest for ten seconds, that is fine. Thirty seconds later it might be somebody else. They can learn pressure, cover and protecting the goal in very simple language without one player being labelled the defender.