Hilarious adventures of some very British children (Edwardian era) who try very hard and very earnestly to be good, with disastrous results. 

The Psammead kids and the Railway Children are believable children and nice, even though they all get into stupid scrapes a lot. They seem to learn from their mistakes. But the Bastables, in the Treasure Seekers and Wouldbegoods, are either sickly-sweet dogooders (Dora and Daisy) or kids who just don't think, and don't seem to learn anything (the rest of them). They get into scrape after scrape - unlike Nesbit's other children who have some legitimate good times - and then the Bastables get sent to bed and that's the end of it, until they do something dumb the next day. Also, this book is purportedly penned by one of the Bastable children and so there is a lot of liberty taken with grammar, history, etc. 

The book is considered better than The Treasure Seekers. Lot's of fun. A perfect match!