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.