little girl; young girl
Refers to a female child, typically from toddler to early elementary school age. Neutral in register but can sound slightly formal or literary compared to 女の子.
A little girl is playing in the park.
The mother was soothing the crying little girl.
その幼女は母親の手をしっかり握っていた。
The little girl held her mother's hand tightly.
Every morning, she drops her little girl off at daycare before going to work.
少女 refers to a girl from roughly elementary school age through her teens, whereas 幼女 refers to a much younger child, from toddler to early elementary age.
幼児 is a gender-neutral term for a young child (boy or girl), while 幼女 specifically means a young girl.
counter for people
Compound of 幼 (young, infant) and 女 (woman, female). The reading ようじょ is the standard on'yomi combination.