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You should keep the medicine cabinet somewhere the kids can't reach.
Literal
Medicine-box [topic-は] children [poss-の] hand [subj-の] not-reach place [loc-に] keep-[ておく→とく] direction [subj-が] good [assertion-よ].
Casual register with two contractions worth noting: 置いとく is the colloquial reduction of 置いておく ('keep it placed'), and ~方がいい ('had better') combined with ~よ gives the whole utterance the flavor of friendly advice. 子供の手の届かない is a relative clause modifying 所 — literally 'the place where children's hands do not reach' — a fixed way of saying 'out of reach.' Note the ga-marked subject 手 inside the relative clause, typical of subordinate clauses where が is preferred over は.