Great photo! I don't immediately spot any children of William Edgar Oliver and Elsie Jane Elizabeth (nee Johnston) which is a shame as I have seen several of pictures of then that would have been great confirmation.
However, I think possibly the eldest grandchild of Mary Johnston (nee Stoker) was Mary Alexandra Johnston b.1892 d.1954, and she was first daughter of William John Johnston b.1864 d.1932 (William being Elsie Jane Elizabeth Johnston's sister).
There is a picture of Mary Alexandra in one of the online tree's and for me the picture certainly has a similarity to the eldest girl in the family photo.
Lots more digging and checking required here to piece it all together, but initially I would think this could well be Mary and some of her grandchildren - exciting!
