Stowaway to the Mushroom Planet

Blurb

Stowaway to the Mushroom Planet is the second in the series of Mushroom Planet books by Eleanor Cameron, and was published in 1956, two years after the original.
This children's book is set in a beach community in California, as well as on a tiny, habitable moon -- "Basidium"—in an invisible orbit 50,000 miles from Earth. The "Mushroom Planet," visited by the protagonists David and Chuck, is covered in various types of mushrooms and is populated by little green people.