You say "go" and each kid has to protect his/her ball while trying to kick the other kids' balls out of the square. If your ball goes out of the square, you're out (for that round). Last kid in the square wins.

Rounds last maybe 1 minute and kids love it. They learn to use their body to shield, use both sides of their foot, use both feet, keep the ball close to their feet, keep their head up while maintaining ball control, etc.