I am getting confused about whether or not there is the accent on the final "a" in "diewczynka." In a previous sentence, I didn't put the accent and I got an "almost correct" because I did not include the accent, but in this sentence, the word is listed without the accent. I am not sure which is correct/if both is correct, and if both are correct, when should I include the accent?
They are the same word but in two different forms which depend on function of the word in a sentence. Have you read basic information about Instrumental? Look here: https://www.duolingo.com/skill/pl/Defining or here: https://en.wikibooks.org/wiki/Polish/Noun_cases#Instrumental
"dziewczynka" [Nominative] -> "dziewczynką" [Instrumental]