I just want to add one more perspective. When our kids were the age of your kids, we thought they were too young to go to Europe, and that we would "wait" until High School when they could appreciate it more. Now they are in High School, and indeed they are going to Europe, and they will appreciate it more fully. But they are going with a school group, which means their mother and I will NOT be there to enjoy it with them. Don't get me wrong. I am THRILLED that they are doing this, and one of the reasons we went for it was to give them that experience of traveling "alone" (with 3 teachers from the school and a tour guide group, but they will have some time on their own in several cities).
However, if I could talk to me back when my kids were younger, I would say, "DON'T WAIT!" Do whatever trips you want to (and can afford) with your kids, NOW! They may not get as much out of it as they would when they are older, but they will get a lot out of it, and different things out of it, and you will be there with them to enjoy it with them. So personally, even if I didn't do a big excursion at every port, it would still be a great trip, and I would be glad we didn't wait.