Just been to the natural history museum, but the whale exhibition seems to have finished which is a shame as interesting for both adults and kids. The queue was horrendous, but if you buy tickets to an exhibition you can skip the queue as they have a time slot (just show tickets/phone to the lady on the stairs outside and they will let you in)-perhaps the Wildlife Photography if you can get there early or late, venom has lots of slots and is the cheapest one.
Wildlife Photography: http://www.nhm.ac.uk/visit/exhibitio...e-year-53.html
Venom: http://www.nhm.ac.uk/visit/exhibitio...hibitions.html
Not done the science museum since I was a kid, so no idea.
British Museum is okay if near Euston/ Kings-Cross, just the free bits, exhibitions look a bit dull for kids.
http://www.britishmuseum.org/
BTW if anyone is near Tring the Rothschild Museum hosts the wildlife photography for free later in the year (usually late summer/autumn I think)
http://www.nhm.ac.uk/discover/wildlife-photography.html