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.
A baby girl was born at the local hospital last week.
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.
女児 refers to a young girl, while 男児 is the equivalent formal term for a young boy.
The everyday counter for people: 一人 (ひとり), 二人 (ふたり), 三人 (さんにん).
More formal than 人 — used for headcounts in reservations, staff lists, and announcements (5名様).
Compound of 女 (jo, 'female') and 児 (ji, 'child'). The kanji 児 is commonly used in formal terms for children.