A girl was playing in the park.
Anime for young girls is popular.
schoolgirl; girl pupil
Specifically refers to a girl attending elementary school. The context usually makes the school setting clear.
The schoolgirls are going to school with their randoseru backpacks.
She gave birth to a baby girl this morning.
This toy is a product aimed at young girls.
女児 sounds formal and is typical of news reports or documents, whereas 女の子 is the everyday spoken word for a girl.
Compound of 女 (jo, 'female') and 児 (ji, 'child'). The kanji 児 is commonly used in formal terms for children.