This quote is significant because it shows that the island is turning from a comfortable and fun place to a place of danger.
The forest looks familiar in the daytime. It is the peaceful place they are used to. It is the sunny side of their dispositions. At night, the island changes. This means the boys are changing.
What they might become in darkness nobody cared to think. They worked therefore with great energy and cheerfulness, though as time crept by there was a suggestion of panic in the energy and hysteria in the cheerfulness. (ch 8)
Things are no longer innocent and peaceful. There is danger ahead. They have reached the turning point where they can see the darker sides of their souls. They now recognize the darkness, symbolically, as the darkening of the landscape at night. Even though things seem to be going well, there is an undertone of trouble ahead.