Neither Pauline nor Alice lived with the grandparents in 1915.
1915 Iowa state census, Grundy Center, Grundy County -
Alex Buchan [54], Elizabeth [48, wife], William [27, son], George [19, son], John [18, son], Emma [16, daughter], James [15, son], Arnett [11, son], Theodore [9, son],
William Buchan [50, brother of Alex?, widowed], Ernest [23, son], Florence [23, Ernest's wife?], Anna [20, daughter], James [16, son], Lyle [15, son], Nellie [11, daughter]
Sara Buchan [66, who was she?]
https://www.familysearch.org/ark:/61903/3:1:3Q9M-C9PN-S4ZG?i=1988&cc=2240483 [and subsequent images]