"What is it, a girl or a boy?"
Translation:Co je to, děvče nebo chlapec?
One is a quite special word. It is indeed "it" https://prirucka.ujc.cas.cz/?id=ono but it is not used too often in the regular sense in nominative. To some extent it becomes defective in modern Czech.
Please understand that the English "it" is pretty overloaded and means something else in different use cases.
Namely, in the extremely common "Co je to?" / "Je to X" / "To je X". You cannot use "ono".
Ono would be used when speaking about some particular thing represented by the "it".
Vidíš to dítě? Ono si hraje.
Do you see the child? It (he/she?) is playing.
Then ono can be used in specisl subject-less sentences like: Ono prší. It is raining. However, here it is used more as a stressing particle rather than a real subject.
Finally, you can use the different ono from the onen/ona/ono triple. Ono dítě = that child.