At long last I have found a reference to Phoebe's death.
This is the entry in The Lethbridge Herald dated 27th July 1921
"Mother and Daughter Drowned.
Regina July 23. Mrs R M Seymour and her daughter, two and a half years old, were drowned at Lac Pelletier, in the Cadillac district, south of Swift Current, on Saturday night. Both bodies have been recovered"
Now to find out their place of burial.
Regards
Nell