If it is the Biggles from WE Johns's books he must be a good guy!

Who is this "They" and who wrote them? Childern's Parent or Biggles?

I think it can compared to Hitler burning books to take away knowledge. Or maybe just a combination of ignorance, superstition and fear in that tribe that broke it.

Oh did you noticed the someone who written the messages must be pretty advanced, because until you get the correct technology, your villagers can't read the writing.

I don't think it would be Biggles, since he shows up again, and he isn't ancient when he shows up.
I've always thought that the writing was from the villagers that once inhabited that side of the island. This would include the children's parents.
And yes, they were very advanced. When they were there, they probably had all the tech, but years of neglect and disregard changed that.

That advance is to be able to clear the vines. It's only because the vines are covering the writing that the villagers can't read it.

I have to put in a theory now:
The parents were afraid of the children getting hurt by the gong (since it's so unpredictable), so it was dismantled. The gong was stolen from Biggles' tribe (maybe VV3?), and he wants it back.