Keep all the medicine in a place the kids can't reach.
Literal
Medicine [topic-は] all child [poss-の] hand [subj-の] not-reach place [loc-に] put-away-[keep-ておく][command-なさい].
しまっておく stacks two auxiliaries: しまう ('put away, store') plus ~ておく ('leave in a state of preparation'). The combined effect is 'store away and keep stored.' 全部 here scopes over 薬 — every single one of the medicines, not just some.