IMO, teach dribbling, rebounding, then more dribbling and more rebounding.
You can get them to run a few screen drills in practice. But, in a game with live ammo, you'll be lucky if they can properly execute it even once or twice.
The kids need to maximize their ability to handle the ball under pressure. Everything starts with the most basic basketball principle - beat the first defender. Then they need to crash the boards, because the average team shooting percentage in 6th grade basketball tops out around 20%.