My kids loved the London Eye - we did it in the daytime & evening (around 4pm as it falls dusk with the crimbo lights coming on will be magic) tickets can be booked online or bought at the site.

A bus tour (double decker) is also a good way to get a quick appreciation of whats where & just how big the place is. A ride on the top deck of a bus down Oxford St to see the lights & Selfridges windows at Christmas are magic!

British Museum is worth a head around the door - good exhibits - free & in a nice part of the city.

I thought the Tower of London was overpriced & very boring - ok if you are into that sort of stuff, swords on walls, racks of ancient guns, etc (zzzzz)

Go to the theatre - ticket booth in Leicester Square for ticket returns for the same day 1/2 price or less. Or the cinema - but personally the atmosphere of the theatre is fantastic. If you want to sample a really British experience go to a panto - warning - sense of humour required & may already be fully booked.

Being 6 - just stood on a platform watching all the tube trains coming & going will be an endless source of entertainment especially for a boy!

China town - for eating & strange things in shops!