N. Europe is the best bet, IMHO and has the following for kids of these ages:
Both easy DIY from the port.
Stockholm: take
DCL shuttle or the HO-HO that picks up right at the pier (get SEK for the Ho-Ho (Swedish $$) before you leave the US--they are not on the euro). Take the Ho-HO to downtown and walk around. You can go into the Palace, etc. but the BEST place was the Livrustkammaren. Founded in 1633, it is also Sweden's oldest museum. Set in the palace vaults, this armory isn't just about weapons but displays some of the world's most magnificent state coaches and coronation robes, even the costume worn by Gustav III at a fatal masked ball. (The king was assassinated at the 1792 ball, and the incident inspired Verdi to write his opera The Masked Ball.) They have a free kit that you can borrow for the kids--it is a box with a fake mouse and y ou find clues throughout the museum. My kids (9, 13) spent 2 hours doing this--super fun. They also have a good dress up place, as well. We got the combo Ho-HO ticket and also took a boat tour. We did not have time to see the Viking ship (on the HO-HO bus/boat line).
Tallin--easy to walk from pier to town and they have costumed persons selling nuts, sodas, etc.--very festive. Plus, you can do archery outside the fort with Red Riding Hood type costumed workers. Large square in center of town to get food, etc. Tallin looks like a storybook village from a fairy tale.
Loved the Baltic cruise--St. P was DD9's favorite spot. Long day with private tour. Too much for a 5 year old. This would be a good day to consider leaving him/her at the kids club. Or, taking on one day and leaving the 2nd day.
CPH has TONS to do for kids these ages, including Tivoli.
I have also been on the other cruises. I think kids would like Baltics better. DCL fjords is fine--we did the following: Stavanger-boat ride to fjord, Movie onboard after lunch, Alesund--walk around town, climb 400 stairs (5 years old were climbing) to lookout on mountain, Bergen-funicular to top of mountain and walk down (can b/c of rain), Museum, Hanseatic league museum (old house) with fishing museum (included in ticket--5-10 yr olds would like this a lot--lots of hand on stuff), Geiranger-bus to Mt D and kayaking--you could do panoramic bus to lower lookouts and boat ride instead. for the West, we did the following--Vigo-bus tour to St ?? Cathedral, Portugal-bus tour to Sintra and all around (adults loved it), Cherbourg (we were at a LE Havre) DDay beaches tour and Rouen (too long and too boring for kids your ages, IMHO). Elaine