The Duchess of Cambridge cast a spell of enchantment over the studios where the Harry Potter films were made Friday on a visit with husband Prince William and his brother Prince Harry.

The three young British royals carried out the official opening of the Warner Bros Studios at Leavesden, north of London, where the blockbuster movies about the boy wizard were produced and which houses the popular Making Of Harry Potter tour.

Kate, who is around six months pregnant, wowed the 500 children invited to the event with their parents by wearing a black jacket and a £38 polka dot dress from Topshop which highlighted her baby bump.

The children are all associated with charities that have one of the royals as their patron.

Kate, along with William and Harry, met film studio executives and inspected some of the props and costumes from movies made on the site.

Vehicles from the Dark Knight Batman trilogy, starring Christian Bale in the lead role, were on show, including the Tumbler -- the movie's updated take on the Batmobile -- and his futuristic bike the Batpod.

William jokingly told Josh Berger, president and managing director of Warner Brothers UK, Ireland and Spain, that he is willing to take over as the star of the movies.

"Christian Bale's getting old and tired...I will stand in if I have to," the prince said.

William, a keen motorcyclist, jumped on the Batpod, leaning forward and gripping the handlebars as Harry and Kate looked on.

He joked: "Where's the start button?", adding "It's awesome, absolutely incredible."