The diamond looks fine until we lose the ball

I would give them one recovery reference. The two wide players do not need fixed defending positions, but they do need to recognise that the moment the ball is lost has changed their job. Something as simple as “can we get back into the game?” can be enough. In training, use a 4v4 directional game and let the team that wins the ball attack immediately. That exposes the same problem over and over without stopping to slide the diamond around. You can praise the wide player who reacts first and gets goal-side or recovers towards the middle. I would avoid building a separate out-of-possession formation at U9. The useful learning is the transition: I was helping the attack, we lost it, what can I do now to help us defend?