"Why do you not raise this girl?"
Translation:Dlaczego nie wychowujesz tej dziewczynki?
"dziewczynki" are "girls" in Nominative (Dziewczynki są wysokie = The girls are tall) and Accusative (Widzę dziewczynki = I see the girls). But it's also Genitive singular. Most feminine words have those three forms identical.
So to understand which form is used, you have to understand what is needed. "wychowywać" (to raise) takes Accusative. It is negated here, and negated Accusative turns into Genitive (only Accusative changes case when negated, other cases stay the same). So as we need Genitive here, this means that "dziewczynki" is singular.