The majority of basketball drills for youth will focus on basic fundamentals, and this is a good thing - the younger and more inexperienced the player, the more the player's coaching should revolve around basic fundamental skills.
The Slide Across the Key is a simple basketball drill that mimics the start-stop, quick direction change a player follows when playing defense on his man. The focus of the drill is to improve the player's defensive slide - refer to man-to-man fundamentals for teaching points on this necessary defensive skill.
Remember to constantly drive home that the player needs to stay low the entire time, in proper defensive stance throughout. It may be a simple drill, but it is also a physically demanding drill - if players do it properly, staying low throughout and sliding as quickly as they can, by the time they finish, their thighs should be burning.
A good way to organize this is to have players in pairs, with each pair at a hoop (if you have 6 hoops in your gym; otherwise, you can use lines on the court that are key-width apart). Have one partner count for the other and note results when you switch players into the drill.
Its too bad defense doesn't generate the appeal that shooting or ballhandling does - it would be nice to see our players out practicing their defense in the driveways some days as well as their other skills.
I have found when I am coaching basketball drills for youth, especially defensive drills, that by going over the top, by playing it as a badge of honor that my players are able to finish a tough drill, it makes them put more effort into it.
So go crazy, be the drill sergeant and get them riled up. Do what you need to do to improve their defense, because as the saying goes,
"Offense wins games; defense wins championships"