Depends on the children, I'd say. If they are even remotely interested in museum-y type things, there are loads of nice little museums in Cambridge:
Fitzwilliam Museum - lots of diff. exhibits
Sedgwick Museum of Earth Sciences (Dinosaurs & Fossils)
Museum of Archaeology and Anthropology (Totem pole)
Scott Polar Research Institute
Museum of Classical Archaeology (Casts of Greek & Roman statues)
Lots of them have fun-pack type things for the kids.
Otherwise, Wimpole Hall and Anglesey Abbey are stately homes which are fun to walk about, and if there was a break in the clouds, both have lovely grounds.
Whale of a Time/Cheeky Monkeys/Funky Funhouse soft play places.
Picturehouse or Vue Cinemas - Toy Story 3 is brilliant!
Ermm.
The Central Library has a great kids section at the back.
If I think of anything else, I'll post again.
HTH.