A mood is a feeling and I think these kids experience the feeling of abandonment. Think about how it might have felt to indeed be a child during WWII. With a dad off to war and a mom having to work and parent at home, these children likely often felt as if all was at a loss for the structure they use to feel at home. Being a child in England, there were probably a limited people to choose from being a small island nation.
When these kids arrived on the island, there are no parents, no authorities, no mentors, and no adults whatsoever to consult with about how to live life. I think Piggy in particular really feels abandoned.
As they spend time together and allegiances real or percieved are broken, children once again experience a mood of abandon from their friends.
Another mood might be loss or hopelessness. These kids lose their hope of home, they lose faith in their leaders, they lose their own previous identities, and that great scar the plane leaves reminds them of their loss of control over their circumstances.
There are many "moods" in Lord of the Flies. To detail them would be exhausting. Suffice to say, Golding's moods are all dark. Even the few lighter moments exist under the umbrella of impending darkness. The sense of wonder and excitement in chapter one is tainted by Piggy's feelings of isolation. Simon's kindness is quickly rewarded by humiliation and sadness ( ex Simon gives meat to Piggy and gets threatened). By the end of the novel the island is pretty bleak. There is terror (the twins being tortured) there is death and finally utter self-destruction. I'm not sure what exact mood you can attribute to this but it's all pretty depressing.
The moods are very bitter at first because Ralph and Piggy are very different and when Jack comes along everything becomes weird and awkward because he and Ralph don't get along ever.