。
Put the medicine somewhere the kids can't reach.
Literal
Medicine [obj-を] children [poss-の] hand [subj-の] not-reach place [loc-に] put-[command-なさい].
The relative clause 子供たちの手の届かない uses a possessive の to mark the subject of the embedded clause — a classic alternative to が in prenominal clauses, sounding slightly tighter and more formulaic. Combined with ところ ('place'), this gives the natural Japanese idiom for 'where (someone's) hands cannot reach,' a high-frequency way to say 'out of reach.'