throe

万次郎達は誰も住んでいない無人島でなんと墓石を見つけるのです。
When Denzo told the others about what they had seen, the gathering grew solemn.
"That could be our fate," Toraemon said.
"I have thought that also," said Denzo.
Everyone turned to Jusuke, who heard none of this conversation. He tossed and moaned, in the throes of fever.
Tears streamed down their faces. None of them wanted to have to bury any of their friends.
万次郎はその時14歳、Denzoは船長です。
"throes of fever" は初めて見る表現ではありませんが、この際しっかりと憶えたいと思います。
・Merriam-Webster's Learner's Dictionary: (Defined for Kids)
1: PANG: death throes: throes of childbirth
2 plural: a hard or painful struggle: a state in the throes of revolution
・Vocabulary.com: severe spasm of pain: “the throes of dying”
子供の本と言っても私の知らない単語・表現は結構あります。しかし、たいていの場合辞書を引かないでも意味が分かるような書き方がされています。それで子供は本を読むだけで自然と語彙が増えるのでしょうね。