Every player who is present by the mid-point of the first half should play at least half the match.  To facilitate this, at roughly the midpoint of each half, the Referee suspends play to permit substitutions.  The Referee chooses a convenient stoppage if at all possible.  If a player leaves the field because of an injury, the player may be substituted for, but in that case, the player may not return until the next substitution break.  The "quarter" in which a player is injured and leaves the field counts towards that player's half-game, and not to that of the substitute.

For U9: Any of the other players may change places with the goalkeeper, provided that both (a) the Referee is informed before the change is made and (b) the change is made during a stoppage in the match, such as a quarter break or half time.