There's a great deal of irony in this quote by Piggy. The idea that grown ups are not afraid of "the dark," (fear in general), bespeaks of an innocence on Piggy's part. Of course grown ups are afraid of things. To children, it might simply be called the dark. But the "dark" to adults could be any number of things.
When Piggy asserts that instead of being afraid, grown ups would have tea, the absurdity is complete. Thinking of the circumstances which brought the boys to island, reminds the reader how ironic Piggy's quote is. A war is being fought "out there," hence their plane is shot down. It's precisely because grown ups are afraid and don't talk that the boys find themselves in their predicament.
One could argue that the boys were actually more successful than adults with respect to their plight. They did organize, they did try to get rescued. They got off on the right track, but, perhaps things went astray because they tried too hard to be more like adults.
Perhaps they might have been more successful had they not tried to be more adult.
It is somewhat obvious that piggy is wrong. I know that there have been many times in my adult life when I have been afraid and in those situations I didn't have tea and discuss. Remeber that there is a war going on at this time much like the previous poster has stated. Adults are not much different from the children in this novel.
I think what Golding is showing here is that no matter what age, humans have fears and a sin nature. There are many sections of this book that show this nature in the children(the murders). The war outside is the example of the adult sin nature. This novel says that we are all born with this nature but we deal with it in different ways. Not all humans act on this evil or sin nature. Some conquer this feeling with a sense of what is right and wrong. Ralph seemed to be one who sensed what was wrong and tried to lead the others away from it.
What we see in this quote is a child's hope that things won't be as bad when they grow up. What is unfortunate is that at no matter the age, we will all have to face fear and decisions on what is right or wrong.