As is often the case, I can recall only some details about the book, not the main plot itself.
There were several sets of children on Earth in the near future, separated from their parents for a long time.
The parents had jobs in space/another planet. The children wanted to see them.
They built a ship - forged a purchase order or something for an asteroid what had been captured and was on earth, then more hacking/forging for laser time to hollow it out, outfit it, and so on.
They went off and found their parents, but there was another threat and they had to leave.
The parents' base(?) was isolated and armed with 10 laser weapons of some kind, called, I think "p.o.w's" which fired a targeting laser and would then fire if the targeting laser detected something.
The children stole/were given three of them and attached them to the asteroid ship.
They when went off and...that's all I remember.
This would have been read as a library book in the middle to late 80's. I've a vague feeling it might have been a short series of possibly three books. Equally likely to just have been one, though.
Been trying to remember what it was for a few years now.