I am not sure you are going to find anything of that sort.
You may, however, try looking for private tours, perhaps linking up with others on your cruise (If you don't know how to do that, email me at firstname.lastname@example.org and I'll tell you). Often you can save a little money doing it that way...Of course, some folks may not want to do a small greou tour with children...but it likely depends on the ages of the kids and their capacity to tour...
In some ports, there are tour companies that do their own larger excursions where you can just sign up to join a group.
Some ports there are Hop-on-hop off buses that may allow you to tour cheaply.
Europe can be one of the lowest cost places to visit on a cruise ship because you have a great deal of freedom to create your own excursion since resources are available such as cheap and easy to use public transportation, lots of free sight-seeing, cheap or free admission for children into museums, and flexibility in meals.
What ship, dates and ports? How much research have you done on the ports? The only European port where you will need some sort of excursion or pre-arranged tour is St Petersburg unless you spend a lot on personal visas for each person.