엄마, 아빠 일 나가니까 애 잘 보라고 했어, 안 했어?응? -> "엄마, 아빠 일 나가니까 애 잘 봐" 라고 했어, 안 했어?응? Didn't I say "because we go out to work, you take care of the child" because, but stress the cause 가방 좀 열어보라니까 싫다니까요 here similar stress the cause that what teller said before. very reasonably I guess it is repeated so teller added ~니까 to stress that if I change those sentence as first sentence form. 가방 좀 열어보라고 했으니까 (가방 좀 당장 열어)! because I did say open your bag, (will you open the bag now!) 싫다고 했으니까 (더이상 묻지마)! because I did say NO, (will you do not ask again!)

as far as i know 니까 means because for example 지금 바쁘니까 나중에 전화해 주세요 I am busy now so call me later. 지금 피곤하니까 우리 영화 내일 봐요. I am tired now so let’s watch the movie tomorrow Q. 이거 왜 샀어요? = Why did you buy this? A. 맛있으니까. = Because it is delicious. (반말/casual language) hope its useful