Boy
noun
A man child is a boy. There's a の particle stuck in the middle of this word. Here it just means the noun before の is modifying the noun after. But for now, all you really have to remember is that 男の子 means "boy."
おとこのこ
If you know the kun'yomi readings of both of these kanji you should be okay. You have two separate single-kanji words put together by a の. That's why it's the kun'yomi reading - they're separate.
２０１８年に男の子が生まれました。
A boy was born in 2018.
おめでとうございます、お母さん。かわいい男の子ですよ。
Congratulation, mom. It's a cute baby boy!
あの元気な男の子は来年一年生だ。
That energetic boy will be a first-grader next year.