I'm told kids tend not to have a great time in Paris, and I can see why. It really isn't very kid oriented at all. It is more about atmosphere and food that most kids aren't ready to enjoy.
London is a good choice.
There are a lot of really pretty small towns in Spain that people of all ages can easily appreciate. For example, who wouldn't be amazed by the Alhambra in Granada? When I went, even five year old visitors were in awed silence. If you went that route, you could also go through however many you choose of Cordoba, Seville, Toledo, and Segovia for a day or two each (maybe more in Seville), and also spend a few days in Madrid. I think anyone would have fun in these places, and I feel it may not be as harried as traveling in Italy with family in tow (I can see that being stressful - especially in Rome - but maybe that's just me). Just another option to consider.