The sky's the limit...
But I think retention will depend on the player, and it will start with the coaches and their ability to explain, and drill correctly. As long as you practice, practice & practice some more your defensive strategies, they should be fine. Don't let a player nod his head or say that they understand, make them show you in practice that they really understand by doing.
The other thing to keep in mind, as you probably know already, is that no matter how good or talented these guys are, they're 11 years-old - they're NOT Pro's... Even the BEST players at 11 make bonehead plays, have brain-farts, and make big errors (mental & physical) on both sides of the ball... Plus, unbelievably, sometimes they all act like 11 year-old kids, too!