Mothers must protect their children's teeth from cavities.

Literal

Mother [topic-は] children [possessive-の] teeth [object-を] cavities [from-から] protect-[benefactive-てやら]-[must-なければならない]

守ってやる uses ~てやる — doing a favor for someone below you or in your care. ~なければならない adds obligation. 虫歯から = 'from cavities' — から marks the threat.