What happens when three siblings who basically want to be good and please their mother suddenly lack all supervision? They have more adventures in one summer than the rest of us have in a lifetime. Despite a little contrivance, the book is a fun read even for adults. When the children err they do their best to make things right with the adults around them. They wish to help others even at their own expense, yet still retain the essence of normal, sometimes-squabbling siblings. All in all, it's a wholesome read suitable for any age, although the younger set may not understand some of the British references and speech mannerisms.
The Masterpiece Theater version of this story very closely followed the book. If you enjoyed the simple charm of the movie, you'll enjoy the book just as well.
Do not confuse with The Boxcar Children! Two completely different stories.