When I read this story originally as an 8 year old, I loved it - it's one of the few children's books I still own in my mid-30s and which is read to baby cousins, godchildren, etc. This book appeals in two ways:
If you are a parent/adult fretting about how to keep up the momentum for a boy who has gone through Harry Potter, Darren Shan, Anthony Horowitz and even the late, excellent children's author Douglas Hill, and who is now drifting ominously back towards the idiot box in the corner or the latest mega-gore-slaughter-death IQ-murdering computer game, then find this, Andre Maurois' finest hour. It's got the fat boy and the skinny boy, plus exploration, adventure - and enforced separation.
If you are a parent/adult fretting about a girl who is only 10 but who reads Cosmo during morning "playtime" and can quote Bridget Jones/Marian Keyes/Sophie Kinsella verbatim and who is already talking about diets and thighmasters at age 7, then I would read this. The title - Fattypuffs and Thinifers, will lock-down her attention like the offer of letting her have a sleepover, and though the main two protagonists are boys, the character of their plump mum will strike a chord. You can use this book to discuss body image with your daughter/young girls in a very non-confrontational way.
I realise that sounds a bit heavy for kids, but it makes them and you think - what would happen if the UK were strictly segregated along lines of weight? BMI 30+ you had to live in ABC and -30 XYZ? The story appeals equally to girls and boys and makes a serious point in a witty, funny way. I would also check out: How to Live Forever (though that is more for 2-6 years) by Colin Thompson, The House That Sailed Away by Pat Hutchins and The Phantom Tollbooth by Norman Juster(all of which I have also reviewed). All these books deserve to be in every school library and every child's bookshelf. They are engaging, entertaining, well-written, witty, fun and funny. They also subtly engage children's thinking ability and help them to reason out moral quandaries.