Just remembered a story book on this: Madam How and Lady Why by Charles Kingsley . Here is Yesterday’s Classics’ description but it may be free online (check Project Gutenberg and others)
Introduces children to geology through conversations about earthquakes, volcanoes,
coral reefs, and so on. Encourages children to wonder about the distinctive features of
the landscape around them and how they came to be the way they are. Ages 10-14
295 pages
It is a bit above your kids ages though, so not sure it will help in this instance!