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The uni library has a lot of Egyptian stuff and lots of stuffed animals, some dinosaur exhibits etc; it's ok for an hour or so. There's also the big IMAX in the Printworks - there might be something on there for them (and it's not too far from Wagamamas). There's the indoor ski slope the Chill Factor out at Trafford you could get the bus / train out there (it's next to the Trafford Centre) that would take up a chunk of time. Also a trip out on the metro to altrincham at the end of the line takes about 20 mins and that takes you right to altrincham ice rink. Good luck finding something to entertain them. Btw they are going to love the Lion King
Some good ideas above Science and industry Museum. Take the train to Liverpool as plenty going on Watersports centre, Albert Dock a few museums, all the beatles stuff allways plenty of tourists knocking about