My two cents worth:
The photo was taken on 4 March 1918.  This was the anniversary of Baby's birth.  Baby was an older child, probably the first born, and deceased.  The photo was taken to commemorate his/her birthday and the younger children, Dora and Glyn, were posed with reminders of Baby - the locket, the silver mug, and possibly the bear.  It was sent to Daddie who was away from home, presumably in the army.  Later, a note was made on the photo to indicate that, at the time it was taken, Glyn was 1 year 5 months old.  This would mean he was born in about October 1916.