British Museum - The British Museum has just a boggling collection of fabulous goodies from the ancient world - Assyria, Mesopotamia, Egypt, Greece and Rome, plus prehistoric Europe. The museum is free, and there's lots to explore, but start early in the day, or better yet, come back more than once.
Where to start? At the information desk in the Great Court, pick up Family Trails, such as Sailing on Nile (ancient Egypt), Travelling in Time (ancient Greece), Hunting for Dragons (dragons around the world).
The Ancient Egypt galleries (ground floor, level 3) are filled with mummies (including mummies of cats, crocodiles and baboons), sphinxes, tomb artifacts, and colossal statues of pharaohs. Don't miss the Rosetta stone, important for school reports.
The "biggies" from ancient Greece are the Parthenon sculptures. Marble friezes show the Panathenaic Festival, an amazing procession of horsemen, chariots, galloping horses, musicians, animals, elders, gods Athena, Poseidon, Apollo, Artemis (and more).
Next to the Parthenon gallery is a "hands-on room" with replicas of the friezes kids can touch.
Personal favorites are the amazing Assyrian reliefs and winged bulls and Sutton Hoo treasure from 7th century Anglo-Saxon kings - golden weapons, helmets, swords, drinking horns and silver bowls.
Spend some time in the Great Court and Reading Room (which has a magnificent blue and gold dome). In the Great Court is a cafe, and picnic tables if you brought your lunch (like all the British school children).
To scout out the museum in advance, look into the British Museum website and check out the calendar of activities for kids at the British Museum
Coram's Fields - This park is unusual in that adults can only enter the park if they are accompanied by a child. Once the original site of the Foundling Hospital, Coram's Fields is now a wide spacious playground with swings, slides, and lots of climbing structures. Kids will get a kick out of this London playground.
The Foundling Museum (Brunswick Square) - Right next to Coram's Fields is a new museum that tells the story of the Foundling Hospital. Established in 1739, the Foundling Hospital was a home for abandoned children, and also an art gallery for British artists such as Hogarth and Reynolds, and concerts by Handel. Explore the museum with a children's guide book or drawing activities, listen to an audio tour with poems by kids, dress up in 18th century kid's clothes - this museum is a real eye opener.
London Canal Museum - Kids interested in boats and navigation will have fun in this small museum, located just east of King's Cross station. Here you can find out about the canals that were the main source of industrial transportation from the 19th and into the 20th centuries. Step into a full-size narrowboat (whole families lived on these boats), check out the exhibits of horses that pulled the canal boats along the tow paths, and go out behind the museum to see narrowboats moored in the water.
Russell Square - Large tree-lined square, with benches, lots of grass to flop down on, and a cafe for lunch or a snack. This is the perfect oasis, when kids need a place to run around or you'd like a picnic spot (there are sandwiches shops close by with everything you need for a picnic).