One place to amuse kids (both big and small) in London is Hamleys. The major London toy store sits along Regent Street, just off Bond Street, which is the one of London top end shopping belt.
Hamleys claims to sell "the best toys in the world". When I visited it some weeks back, it does have some items that I've never seen before. I was very much tempted... Anyway, though much of its merchandise can be found in Toys R'us, the winning key is the layout and the well trained staff. While you'll face a sourly cashier auntie at Suntec's Toys R'us, Hamleys staff specialises in a particular set of toys. There are booths within the store manned by staff who will demostrate how to literally play the toy. There is this staff member who will demo magic tricks and his booth is always crowded. Another entertained the crowds by flying toy planes and helicopters above the customers heads. It's amazing, not to mention the comfortable sofas in the basement for customers to 'sample' the newest XBox 360 games.
When I was there yesterday (hey, I'm just walking past that place), there's this pink limo parked outside. Lucky kid.