How do you coach runs across a defender rather than away from them?

No, I do not think it is too much detail if you coach the picture simply. If the attacker runs away in the same direction the defender is facing, the defender can often see everything. A run across the defender can force them to turn, block their path or lose sight of the ball for a moment. I would show it in a 2v1 or 3v2 rather than explaining it for five minutes. Let the passer and runner see how the passing lane changes. A useful cue is: “Can your run make the defender choose between you and the ball?” At U11, movement can become more intentional without becoming scripted. Teach what the run does to the defender, then let players decide when that run is actually useful.