Demonstrative pronoun 'that child/kid' — refers to a person (usually a child or young person) known to speaker and listener; 'あの娘' specifies a young woman.
あの子はとても人懐っこくて、すぐ近づいてきた。
That child is very friendly and came right up to me.
あの子は隣の家の新しい生徒ですか。
Is that kid the new student from the house next door?
写真のあの子は誰？クラスメイトの一人だよ。
Who is that kid in the photo? He's one of my classmates.
Archaic: used as a second-person form (addressing 'you') in historical contexts such as brothels or by senior prostitutes when speaking to servant girls; not used in modern everyday speech.
時代物の台本で、老女将があの子、早く支度をしなさいと言う場面があった。
In a period drama script there was a scene where the madam says, 'You there (ano-ko), hurry up and get ready.'