Hi thanks for using our forum. What age group are you coaching or inquiring about? That is a tactical question. All players can cross center line. There is no rules that a player or so many players have to be in their half of field. There are coaches that will force kids to stay on their half and not allow them to pass the center line. That is more of a tactical formation or red tape. Depending on age this is hindering the experience and taking the fun of soccer away.