A number of customs have arisen in youth soccer that are not strictly in accordance with the Laws, but have evolved because they emphasize good sportsmanship amongst young players.
One such custom involves injury. When a player is injured on the field, and the ref blows a whistle to stop play, players will generally stop where they are, and kneel or sit on the ground. This keeps teammates from clustering around the injured player so the referee or coach can tend to them.
Another custom is that when an injury occurs and the ref does not blow the whistle for play to stop because he or she has not noticed the injury, the team holding the ball will often kick it out of bounds. This stops play whether the referee has noticed the injury or not. It helps call attention to the injured player.
The flip side of this is that the team which then restarts play with a throw-in, throws the ball back to the opposing team. Its good sportsmanship on both sides, and is especially good practice with youth players as it avoids the official restart method, which is a dropped ball which is often a great opportunity for further injury.