We also cruise and travel in Europe with our kids and have had wonderful experiences. When we go this summer it will be our fourth trip there in two years. The first trip was a few days in Copenhagen followed by a Princess Baltic cruise when our daughters were 12 and 6 years old. The second trip was 10 days in London at 13 and 6 followed by two weeks in Provence, France at 13 and 7. This summer we are spending a few days in Rome prior to a Celebrity Mediterranean cruise to Greece and Turkey and the girls will be 14 and 8. We have been traveling with our daughters since they were infants and there are very few places were I would hesitate to take them. We may have to adjust our itinerary and the pace of our sightseeing in order to accommodate them but I believe that it is worth the sacrifice in order to introduce them to the world.
In terms of the timing of your cruise I would recommend trying to go during school holidays. One of the things that my daughters like most about cruising is being able to meet other kids their age. When we cruise they do all shore excursions with us and eat dinner with us but on evenings and on sea days then they can spend as much time as they want in the kids clubs. On a European cruise they will also meet many kids who aren't North American and that is a learning experience in and of itself.
I would also suggest that in order to prepare them for the cruise find some books for them to read about their destinations ahead of time. There are a number of DK Eyewitness books that would be appropriate and there is also the Magic Tree House series for a start which has books and research guides about Pompeii as well as Ancient Greece.