Or you could say that the "hole" in the doorway is to represent the missing father. The one sitting with his mother is the youngest and hasn't left home yet (perhaps emotionally). The oldest one, sitting outside the doorway, knows he has to stay and look after the mother, the one on the far left has divorced himself from the group and is getting ready to leave and the young troublemaker is the one with the bike.

Or you could say, the photographer said, I don't want you all looking at the camera so you look here, you look there etc.

Michael