There were a group of kids (3 or 4) and they were joined by a Chinese girl from a well off family. I remember they visited a plant and something about flies buzzing in their brains at the end where she betrayed her shipmates or something. There was something about having 'encoded cards' that they got on the black market.
Your description partially matches Nicolas Fisk's Starstormers which came out in 1980 (so the timeline is approximately correct).
- It had four kids who built a spaceship out of a hollowed-out asteroid so they can join their parents in space
- one was a Chinese girl, named Tsu
- no flies, but the story involved fighting against semi-sentient "dust" for part of its plot, which was motile and behaved like insects if I remember correctly.