This was the first science fiction book I ever read, I found it in the public library when I was 12 years old, was intrigued by the title and took it home to read. I enjoyed it immensely and went on to devour Heinlein's other kid's classic, "Podkayne of Mars" which I thought was equally wonderful.
Thirty-five years later, Kip, Peewee and the Mother Thing have lost none of their charm. What I found most interesting about this book, however, was how very much things have changed since this it was first published: the story begins with Kip's attempting to win first prize in a soap slogan contest (he sends in 5,000+ entries) the grand prize for which is a trip to the moon. He doesn't win the trip but he does win Oscar-the-Traveling-Spacesuit, which turns out to be the best prize after all. Back to the contest: Kip can send in the actual contest form included with each bar of soap or he can send in a "reasonable facsimile." He entertains the idea of photographing the form 5,000 times before deciding that's impractical, so he settles instead for collecting the forms from the people who have bought the soap. I kept thinking, so why doesn't he just use the photocopier when it occured to me there weren't any photocopiers when this book was written. There weren't any intergrated circuits either, never mind microprocessors, which would have been necessary to achieve colonization of the moon in the first place. I also found it interesting that although when Kip graduates from high school he is reasonably well-educated in sciences and math (thanks to self-teaching -- and a diatribe from Heinlein on what he considers to be the sorry state of 1950's high school education) because he doesn't get a scholarship, he is faced with having to work while attending the local (read mediocre) state college. This book predates the arrival of college loans and grants as well.
I did like the fact that the other main character in this book is a girl -- it was good of Heinlein to provide his readers such an intelligent and resourceful role model.