Soccer's rules are simple on the surface. But understanding them as they apply in a game can be difficult, especially for kids. The offside rule is one of the more difficult rules for kids to understand because it may seem straightforward at first, but it is actually a nuanced rule. Demonstrate the offside rule in a practice with your kids, so that they can see first-hand how the rule is applied in the game.
Demonstrate the offside position to the kids. To be in an offside position, the offensive player needs to be closer to the opposing team's net than the second to last defender, and must be closer to the net than the ball. Have two kids act as defenders and one as the offensive player. Demonstrate that the offensive player cannot pass the second last defender until the ball has past the defender.
Explain that a player is only called for an offside offense if he is involved in active play. Explain to the kids that this means that a player can be in an offside position but he does not commit an offense unless he touches the ball, interfered with an opponent or otherwise gains an advantage by being in the position. Have the players demonstrate examples of gaining an advantage to make the example clear.
Explain that offsides are judged at the moment that the ball is kicked. Kick the ball past the defenders and have the offensive player run past them after you kick to demonstrate how this rule works. Even if the player runs ahead of the ball she is still onside if she was onside at the moment the ball was kicked.
Demonstrate examples of instances in which a player cannot be offside. A player cannot be offside in his own end of the field or when receiving the ball directly from a corner kick, goal kick or throw-in.