。
Tears ran down her cheeks.
Literal
Tears [subject-が] her [of-の] cheeks [object-を] tracing flowed.
伝う ('to trace, follow along') describes motion alongside or along the contour of something — here, tears tracing the line of the cheeks. The を marks the cheeks as the path of motion, a use of the object marker reserved for verbs of motion through or along a space. Paired with the plain past 流れた ('flowed'), the sentence captures the gentle downward motion of weeping with a quietly literary feel.