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Museum of Science and Industry (if they're into that sort of thing)
Taking your kids to the library might sound like a crazy idea but bear with me a moment. Check out the John Rylands library on Deansgate. From the outside it looks like a gothic church, you enter through a very modern gift store / cafe, walk up some stairs and then suddenly ... you're in Hogwarts! Well, not quite, but it's an amazing Victorian building with lots of corridors and rooms and strangely-placed staircases and all that sort of thing. And the main reading room really could be straight out of Harry Potter.
Thank you, I'll look it up. They are both still loving Harry Potter so that could be a great idea.