An excellent book for young people about geology is How the Earth Works. It contains a few demonstrations on how volcanoes work. (Farndon, J., 1992, How the earth works: The Readers Digest Association, Pleasantville, NY, 192 p.)
There are many books on volcanoes designed for kids available online these days.
I checked on Amazon and found many many results.
One particularly good one is Volcanoes: Earth's Inner Fire, by Sally Walker. Published in 1994 by Carolrhoda Books
Category
Studying Volcanoes / Volcanology