With "nie słyszę" you use the genitive.
"Słyszę dziewczynkę. Nie słyszę dziewczynki" = "I hear the girl. I do not hear the girl"
"Słyszę dziewczynki. Nie słyszę dziewczynek" = "I hear the girls. I do not hear the girls"
(for the declension see: https://en.wiktionary.org/wiki/dziewczynka#Polish)
I'm British, and from learning languages at school and being told English is harder I am now finding that hard to believe. "This" is always this in all forms. Cat is always Cat, unless there are 2 or it's a kitten, of course. A boy is always a boy doesn't matter what form. So, to the majority that read this, do you think English is harder than other languages to learn? Just curious :)
The main difficulty, I believe, is the inconsistent pronunciation.
See https://simple.wikipedia.org/wiki/Ghoti#:~:text=Ghoti%20is%20an%20artificial%20word,in%20women%20%2F%CB%88w%C9%AAm%C9%AAn%2F%3B%20and for a pair of somewhat overstated examples.
As I wrote. You have the verb "słyszeć" and it takes Accusative. It's negated here, and negated Accusative = Genitive.
The Genitive singular form is "dziewczynki". The Genitive plural form is "dziewczynek". Therefore this sentence is undoubtedly in singular.
Most feminine nouns have the following forms identical: Nominative plural, Accusative plural, Genitive singular.