Greenwich Observatory -- Prime Meridian

Greenwich Observatory is famous for being the reference point for 0 degrees latitude, aka the dividing line between the Eastern Hemisphere and the Western Hemisphere.  The museum at Greenwich is worth a visit.

The Observatory dome still houses a telescope that you can take a peek through:

The rest of the compound sits on a hill to the southeast of central London.

Nice view of the O2 Arena (formerly the Millennium Dome)

And of the Canary Wharf district:

 Here is the famous Prime Meridian line, embedded in the ground:

 It even comes with instructions

 Along the marker, it shows the locations of various major cities.  A bit disappointed that Philadelphi doesn't make the cut, but that could be due to lingering resentment over the unpleasantness in 1776.

 It is obligatory to have a picture of yourself straddling the line.

 Curious scultpure atop the line. 

 There's also an Astronomy Museum, which may be the only one in the world without an IMAX theatre.

Price: £7
Getting There:  Bit of a haul on the Tube system, although the ride through Canary Wharf on the Docklands Light Railway (DLR) is impressive.  You can also get there by tour boat, which is a pleasant ride and the tour guides on the boats are hilarious in a very British way.
Kid-Friendliness:  Ok, but not great.  It's largely an historical site
Italian Tourist Factor:  Medium Risk of Italian tourists