Book report

I have three daughters, and two of them go to elementary school.
Their teachers gave them a lot of homework for their summer vacation, and one of them their tasks is to write a book report. In fact, writing a book report is one of a popular homework assignment in Japan.
My daughters like reading, but they seem to struggle with writing their book report.
I think it's hard for them to write something logically; to write who, what, how, why, where, when.
How do I help them improve their skill in writing logically?