Some things in London that might appeal to kids:
The London Dungeon. My guide book describes it as "not for the squeamish"
London Zoo
Boat ride to Greenwich to see where time is created.
Tower of London-- a real castle with moats and all that, an armor museum, and the small courtyard where Henry VIIi chopped off the heads of his wives.
Day trips:
Windsor Castle. Another real castle.
Winchester, Stonehenge, & Salsburry
Rye--- way off the beaten track. A cute old village & major sea port hundreds of years ago, now 2 miles inland because the weight of the glaciers in the ice ages pushed the land down and now that the ice has melted, the land is rising back up toward where it should be, making the English Channel a little narrower than it was hundred of years ago.
Visit for lunch or stay overnight at The Mermaid where Queen Elizabeth I stayed some 600 years ago before setting sail from Rye to visit France. Visit The Mermaid's web site & look at the rooms. The ones that look like they were designed by a drunk really look that way. The Mermaid is a classic example of early 15th C half timbered buildings which were a reaction to the prevailing formalism of the first wave of neo-classical archeteture. While neo-classical buildings feature symmetry, half-timbered was the opposite-- floors are not level. Room corners are not square, ceilings are not a uniform distance above floors.