。
The law requires us to send our children to school.
Literal
Law [by-によって], we [topic-は] children [object-を] school [dat-に] attend-[causative-せる]-[must-なければならない]
Rich grammar. によって here marks the law as the authoritative agent behind the obligation. 通わせる is the causative of 通う ('to commute to, attend regularly') — 'make (them) attend'. The pattern 子供を学校に通わせる is the standard phrasing for 'send one's children to school'.