Since this word is made up of a single kanji, it uses the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
You see a child and walk up to him. Then, you punch him in the face (oh no, how could you!!?). He flies through the air, and he's knocked out. A ref nearby yells that it was a "KO!" (こ). That's a knock out.
Make sure you learn this reading pretty well too. Both readings for 子 are going to show up a lot, and this is how you're going to learn the kun'yomi one.
You aren’t my real child.
When that Russian kid grew up she got better than a salmon at swimming upstream.
Don’t you think my kid looks similar to Beyoncé?