CHILDREN'S fantasy, as a genre, is absolutely booming.

Thanks to one JK Rowling, the popularity of whose boy wizard Harry Potter will soon be immortalised with his own themed world in the US, studios had proof that there were millions to be made from such tales.

For the Potter films, four-and-a-half-billion dollars so far, in fact.

Now that Potter's approaching his final years at Hogwarts - just a few more films to go - Tony DiTerlizzi and Holly Black's books about the Spiderwick estate reach the big screen.

And, for once, a British actor takes on an American role (two in fact), as Freddie Highmore (Charlie from Tim Burton's Charlie and the Chocolate Factory) plays twin brothers Jared and Simon Grace.

The duo move with their mother (Weeds' Mary Louise Parker) and sister Mallory (Irish actress Sarah Bolger) into the Spiderwick family residence in New England. One twin is a bit nasty, the other rather nice, but it turns out that Jared's rattiness is due to their absent father constantly disappointing them.

Jared can't resist the temptation to go off exploring the dusty and mysterious property, and soon discovers that there's a brownie, Thimbletack, living in its walls.

Thimbletack warns Jared away from another of his discoveries - his great-great uncle Arthur Spiderwick's (David Strathairn) field guide to the fantastical world.

Despite the written warning "Do not dare to read this book", Jared inevitably disobeys, awakening the wrath of evil ogre Mulgarath (a brief turn from Nick Nolte), who wants the secrets contained within its pages.

The Spiderwick books have been more successful in America than here, so if you haven't heard of them, don't fret. It's easy to get into this world and suss out what's what, thanks to director Mark Waters' sure hand.

Highmore does extremely well playing two people, and making them slightly different, so that children will easily be able to follow who is who at any particular moment.

And Bolger makes for a delightfully sparky young heroine, taking on an army of nasty goblins who surround the house and who are invisible to humans - unless a hobgoblin has spat in your eyes, or you're in possession of a seeing stone.

The technology required to bring such an imaginary realm to life is generally seamless, particularly the moments when the goblins attack the children at the protective boundary around the house, and we watch unseen hands claw at them really rather viciously.

You won't be able to spot any problems with Highmore's double-act either, but we'd expect no less from Waters, who dealt with a ghostly Reese Witherspoon in his last film, 2005's Just Like Heaven.

Knocked Up's Seth Rogen voices a base character who will most probably turn out to be everyone's favourite - ball-of-fun Hogsqueal.

He munches on birds and passes wind, automatically endearing himself to any young audience.