。
She had her son go fetch the doctor.
Literal
She [topic-は] son [to-に] doctor [object-を] call to-go made-go.
Two layered patterns. 呼びに行く ('go to call / go to fetch') is the purposive に-pattern: ます-stem + に + motion verb expresses purpose of motion. 行かせる is the causative of 行く ('go') — 'made/let [him] go.' Together: 'made [him] go to call the doctor.' Note the causee is marked with に, indicating a more directive 'made him do' rather than 'let him do' (which would use を). Standard household-emergency scenario.