Translation of "Knocks" in Japanese

Opportunity seldom knocks twice.

好機は二度訪れない。

Put the cap back on the bottle in case the cat knocks it over.

ネコがひっくり返すといけないから、ビンに栓をしなさい。

I'd send my kid out on his own for a while if I were you. A few hard knocks at this point might do him some good.

可愛い子には旅をさせた方がいいのよ。今の内に少しは苦労させた方が本人のためよ。