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.
Absolutely! It takes real discipline to achieve but bypasses the issue of trying to translate word for word which isn't possible and can become confusing . A great tip!
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!