Nanny McPhee and the Big Bang

Emma Thompson's Nanny McPhee and the Big Bang is a wartime farmhouse story for 9 year olds, told by an author who keeps stopping to tell you what's going on at the film set.

That's the thing to know about this one. It comes in two halves that take turns. One is the story. The other is Emma Thompson's diary, written on a pile of hay in two feet of fake mud while everybody waits for the rain to stop.

The story starts a generation back. Isabel Carrington is supposed to marry well. Instead she climbs a wall at a garden party to go and look for moles, meets a young farmer called Rory Green, and elopes to Gretna Green to be married by the blacksmith. Her parents cut her off without a penny.

Isabel and Rory have three children, Norman, Megsie and Vincent, and are about to live happily ever after when a war breaks out and Rory has to go.

Then the letters stop coming. And a letter on thick cream paper turns up instead, from the posh sister nobody talks about, announcing that her two children are on their way to the farm.

The diary bits are wonderful and completely unlike anything else your reader will have read. The mud is made of sand and something called bentonite so it never dries out. A cow called Beryl is playing a cow called Geraldine and has refused to come out of her trailer. One of the jackdaws ate a mouse in the night, put on eight grams and is therefore too heavy to be allowed to work.

The storytelling voice talks straight at you and keeps interrupting itself. It points out that falling in love with a pretty girl in a pretty field is, quite frankly, stupid, and you should find out what she's like first. It apologises that the actual Nanny McPhee story hasn't started yet.

It isn't all daft, though. The children's father is away fighting, nobody knows where, and the letters have stopped. That's the engine of the whole book and it isn't softened. A child with somebody away from home might find it closer to the bone than you'd expect.

Otherwise it's warm and daft and full of piglets. Mr Green sat up all night building a machine called the Scratch-O-Matic so that every piglet gets scratched fairly, which tells you the sort of father he is before he ever leaves.

If they're after more children getting on with it while the grown-ups fight a war, they'll find William at War waiting for them. William is outraged that he isn't allowed to practise with his gas mask, and he has already thought of several games you could play with one.