A couple more ideas:

--"Fattypuffs and Thinifers" by Andre Maurois (originally Patapoufs et Filifers, pub. 1941, repr. Jane Nissen in 2001, w/illustrations by Fritz Wegner; 96 pp.). Two brothers, Edmund and Terry, find themselves in the Country Under the Earth, where the two nations of inhabitants, the Fattypuffs and the Thinifers, are at war, over differing philosophies of, shall we say, personal avoirdupois. The boys are forced to take sides in this silly conflict, which escalates into something rather serious, but they help the principals find a way towards mutual understanding and peace. Pointed satire, very funny.

--"My Friend Mr. Leakey," by JBS Haldane, pictures by Quentin Blake (first publ. 1937, repr. by Jane Nissen 2004; 150 pp.). More a collection of short stories (though several of them are linked) than a novel, this book is highly inventive, and just the thing for precocious science-lovers (many of the characters are botanists or physicists or chemists, etc.--my favourite is the physicist Dobbs, who made thousands of pounds yearly from the British railways for carrying underweight luggage--he hooked up hydrogen jets inside his suitcases, which made his suitcases float; an attached electromagnet pulled up the metal plate on the scales, so great piles of weights had to be added to his baggage to get it up to zero. He successfully sued to be compensated for the weight he was not bringing on in his luggage, at the same rate people with overweight bags were charged.)

Several of the stories feature the magician Mr. Leakey, and the utterly logical treatment of the many fantastical happenings gives the book a unique tone. Mr. Leakey has a magic carpet, a pet dragon who breathes fire to grill fish for his dinner, an octopus who waits at the table, and other interesting characters as servants or friends. At one point, he has a costume party at which he magically changes everyone into their costumes: people become an atom of caesium, a comet, a fire engine, an icosahedron, a yak, a tortoise, Shakespeare, etc. Original and fun!