For any kids into their science, the National Museum of Computing at Bletchley Park in Bedfordshire is a must. It has a replica of the Bombe built by Alan Turing during the war years to crack the Enigma Code. It also hosts a rebuilt Colossus, the first electronic computer. It also has a vast range of computer memory devices from car readers to one of the first disc drives. You can call and arrange a guided tour – which is recommended.
The National Maritime Museum has some of John Harrison’s prototype clocks used to solve the longitude problem. This is well worth a visit and is free. 
The Observatory Science Centre at Herstmonceux Castle, the former site of the Greenwich Observatory, hosts courses on astronomy, permanent exhibits on science themes and much more.
Dungeness B Visitor Centre is where you can arrange a visit to a nuclear reactor. What’s more, it is free.
