Our kids loved paris, short flight time and day trip to disney by train possible. They loved the Eiffel Tower, the Metro,double decker carousels, crepes from street vendors and hot dogs with cheese grilled.
Agree with Venice. Barcelona would probably be good. Definitely avoid Vienna - worst place I've ever been with my kids and they've done quite a few cities. Milan would be boring. Prague very fairy tale and some great parks etc if you look around (cobbles are tough on the feet though). Copenhagen - lots and lots to do especially if Tivoli is open. Stockholm great too. Amsterdam and Paris are always fun and easy to get to.