is the oni/ony/ona optional here? or it should not be used at all?
If you would like to use "oni/ony/ona" so you have to write:
ONI = Ano, oni jsou teď malí.
ONY = Ano, ony jsou teď malé.
ONA = Ano, ona jsou teď malá.
Got it, děkuji!
Tip for learning any language: try to imagine the meaning of the sentence with your brain without using words from any other language you speak.