Girl, Young Girl
noun
A woman child is a girl. This word has two kanji with hiragana stuck in the middle. That hiragana is the particle の. This time, it just means the noun before の is modifying the noun after it. 女の子 is a casual word for young girls, like kids or teenagers. Sometimes it can also refer to adult women, but only to playfully or affectionately highlight their youthful or girlish qualities.
おんなのこ
Since there's hiragana in here, you can bet that these kanji will use the kun'yomi readings. You already know the kun'yomi reading for 女. The only thing I have to make sure that you know is 子. When you need to remember the kun'yomi reading of 子, just think about you fighting a child (you're a jerk, apparently!). When you hit her in the face, she flies to the floor, and the ref calls out "KO!" because you just knocked the poor kid out.
白のドレスの女の子がレイチェルです。
The girl in the white dress is Rachel.
この女の子は五才の天才プログラマーだ。
This girl is a five-year-old genius programmer.
わたしは女の子じゃない。一人の大人の女。わかった？
I'm not a girl. I'm an adult woman. Got it?