We've just come back from a day at the Science Museum and I didn't realise how fabulous it was for children. Apart from the sheer space they run childrens lectures in the basement where they blow things up and set others things on fire. Our five year old loved it!
http://www.sciencemuseum.org.uk/
Details of the daily science shows are here
http://www.sciencemuseum.org.uk/visitmu ... shows.aspx