I made the kids who forgot their red pencils come back and properly borrow one.
Literal
Red-pencil [object-を] forgot children [target-には], properly borrow-to come-causative-past [polite-させた].
Causative form. 来させた is 来る ('come') + causative ~させる + past = 'made (them) come.' The causative expresses making/letting someone do something. 借りに来る ('come to borrow') uses に to mark the purpose of coming — a common pattern for purposes of motion verbs. The whole sentence reads as a teacher's recollection of a pedagogical approach.