Blue Planet Aquarium, Ellesmere Port: kids love fish, there's lots of fish...
Manchester Museum of Science and Industry: knobs to twiddle, things to blow up, mess to make, things to crash, trains and space ships to ride on, dinosaur and Dr Who exhibitions to be terrified by. Oh and history of the industrial revolution to make your parents feel smug.
The dungeons in Edinburgh. Just don't take anyone under 6 cos they'll crap themselves.
Alnwick Castle and Garden. Bloody brilliant - Harry Potter moments, dress up as a knight, do scary puzzles, watch owls eating small mammals, discover a whole load of info about how to poison people with plants from your garden (note to self: don't let 8 year old take notes in the poison garden), ride Tonka trucks through fountains and get very wet in fountain maze... You can buy a week long pass and keep going back if you're on holiday...
Trentham Gardens, Staffordshire: Barefoot walk, fantastic play area, woodland walk, hobbit houses, ice-cream parlour and mahooosive amount of consumer options for Mummy and Daddy to be a whole lot poorer...