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; 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 in Hogsmead... Plus, no books captures a child's attention better 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.
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 is quite recent, and stars Richard Dreyfuss and Elijah Wood.
An old tale, dating back as early as 1600. Poor and homeless Dick Whittington becomes Lord Mayor of London, thanks to his cat. This story is based on the legend surrounding real-life Richard Whittington, who was mayor of London three times, starting in 1397.
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.
Also from Disney: there are two versions of this comedy set in London. The original version came out in 1961.
Classic mystery; classic London.
Terrific renditions of the classic tales; a BBC production, which had a long run of screenings on PBS.
For kids too young for Sherlock Holmes, try Sherlock Mouse: this is a fun flick with lots of Victorian London ambiance. There's even an appearance by Sherlock himself (voiced by Basil Rathbone.) There are Mouse Detective books, also.