London is an exciting place for kids to visit, not only because there's so much to do, but also because London is the setting for so many books and movies. Set the mood, and heighten the anticipation for your trip, by reading books and seeing movies set in London-town.
Of course, most of the action in Harry Potter books takes place at Hogwarts and other magical places; but still, every reader knows about the famous Platform 9-3/4 at Kings Cross Station. And in general, the books feel "English": the boarding school traditions; the pub in Hogsmead... Plus, Harry Potter books can be counted on to keep a child engrossed on a long plane flight.
Ditto for the Harry Potter movies; if you have a DVD, you can watch the show on the plane, on a laptop or DVD player.
3. Oliver Twist
Anything written by Dickens is a great choice for helping kids imagine what life was like when all those old buildings and streets in London were 150 years younger. Dickens was a great force for social reform in the Victorian age.
There have been several film versions of this Dickens classic. This one stars Richard Dreyfuss and Elijah Wood.
6. Mary Poppins
A delightful children's movie (and also a book), that beautifully conveys the London era of proper nannies and suffragettes and chimney-sweeps. Some consider it to be Disney's best film.