Jodrell Bank used to be fantastic, but alas is now a minor one hour visit. Bit dull unless you all wear white lab coats. Like the idea of playing footie near the big dish, okay kids five pounds to anyone who vcan hit the dish with the foortball. You could always add a stately home in too, Tatton Park, Lyme Park etc. Stapely Water Gardens is pretty good as an add on too.
Blue Planet Aquarioum – dear, google voucher codes for it instead. Same can be said for Chester Zoo if you fancy that instead.
Manchester is probaby a better bet. Science Museum – interactivegallery is currently closed (that's the best bit). Manchester Museum is fabulous. New Lego Discovery Centre opening soon in Trafford Centre (may already be open). Or catch the tram up to Bury and go for a ride on a steam train at East Lancs Railway (check times so you go steam rather than diesel).
Or go upto Buxton – go down Pooles Cavern, play in the Pavilion Gardens, if it is raining the little museum is pretty good.
Castleton – visit all the showcaves (joint entry ticket makes it cheaper), the castle, eat chips and ice cream.