'I don't know yet' would be good as a reply to someone who asks you, 'When are you going to China? Otherwise, sentence 1 should use 'I'm going' to be natural sounding, 'I don't know yet when I'm going to China'

In general, we use "yet" in perfect tense. But in your sentence, when we say " i havent known yet", it is not completely correct. I think it should be" i dont know yet when i will go to China" it means i dont know the time

I would say "I still don't know when I'm going to China" ;)