We did Paris with the kids a few years ago (aged 5, 9 and 10 at the time) and had a fab time, we did the Eiffel Tower after dark which we all loved and the Luxembourg Gardens and the Botanical Gardens and the Sacre Coeur and Notre Dame and the Louvre and a boat trip up the Seine. We had an airbnb which was reasonably central and walked about a lot absorbing the atmosphere.
We've also taken them to Florence in October and loved it. We had this kids guidebook which is great, has some great ideas for things to look out for, DS had fun seeing all the signs of the Medici and there are modified street signs all over the city as well. We did the Uffizzi and the Pritti Palace (prebooked tickets with an early start), didn't go into the Duomo (queues too long) but did enjoy spotting various things outside. My greek myth obsessed son loved all the artworks. And we had a day trip to Lucca which was lovely (more churches plus a fun cycle ride round the city walls). And we loved the food, Italy is great for food they are familiar with that is still 'foreign'.
The PP who suggested Stockholm is right actually, there is loads of child friendly stuff, we went prekids but now I'm wondering about going back with them!
The important thing to remember is to be relaxed about doing less with kids than you would without them and try and vary your days so there's some more chilled out days as well.
In the UK we are trying to fit in lots of city breaks with them now as well. They loved Edinburgh, Glasgow, Inverness, Manchester, Oxford, York, Bath. We need to go and have a holiday in London soon as well I think.