"She is a girl."
Translation:Ona jest dziewczynką.
In Polish (and in many other languages) nouns change, depending on their function in the sentence. When you use 'Kobieta' or 'Dziewczynka' as a subject, it is in the nominative. When used as an object, it is in the accusative or dative, in this case 'Kobietą' or 'Dziewczynką'.
If I'm correct, it's the same in German, but there only the article changes (ex. der -> dem/den).
Yes, dziewczynką is pronounced somewhat similiar to that. But dziewczynkom is a word that exists too : https://en.wiktionary.org/wiki/dziewczynkom#Polish , but is the dative plural form, and is not relevant in this sentence.
'jest' is the third-person present tense of the verb być(to be), so it means "is". 'But 'je' is a different word with a few different meanings - you've likely seen it on Duolingo as the third-person present tense of the verb jesć(to eat), so it means "eats", which changes the meaning of the sentence :)