An adult book rather than a child's but significant in the history of the promotion of spaceflight. On March 3, 1950 a conference was held about "space medicine". This was one of the first professional conferences to explore how humans might react to the spaceflight environment. This book is a collection of some of the experts who presented at that conference.
Marbarger, John Porter. Space medicine; the human factor in flights beyond the earth. Urbana, IL: Univ. of Illinois Press, 1951.
In fact this may have been the first "popular" book about space biology. Here is an illustration speculating what kind of plants might be on Mars.