If you don't want your little brother to touch it, don't leave it somewhere he can reach it.
If you don't want him to play with it, put it somewhere he can't get it.
この場合、次のような言い方ができますよ。
ーIf you don't want your little brother to touch it, don't leave it somewhere he can reach it.
「弟に触られたくなかったら、彼の届くところに置いておかないの」
to leave it somewhere he can reach it「彼の届くところに置いておく」
ーIf you don't want him to play with it, put it somewhere he can't get it.
「彼に遊んでほしくないなら、彼の届かないところに置いておきなさい」
to put it somewhere he can't get it 「彼の届かないところに置いておく」
ご参考まで！
Put it somewhere he can't reach if you don't want him to play with it.
「触られたくなかったら、届かない所に置こう」は英語で「Put it somewhere he can't reach if you don't want him to play with it.」と言います。子供が話し相手なので、まず命令の方（Put it somewhere...)を先に言うのが子供に聞いてもらえると思います。
触られるのが嫌なら、はじめから弟の届かないところに玩具を置いておこう。
Put your toy somewhere he can't reach if you don't want your little brother to play with it.
カナダ 
