Our kids enjoyed both, but they are very different. I would read
cruiseciritc.com or other reviews of the ports to decide. N. Europe is more of a city-oriented cruise, Med. is more about iconic monuments/ruins of Roman, early Med. civilizations. Both offer a lot. In summer, Med. will be much hotter. In N. Europe, you can even need a sweater in July. What do you like to do? Is there any port you are especially interested in? For a 1st trip with kids of those ages, I would do Med and go to Pompeii (easy, cheap DIY 1/2 day), Coliseum, and then probably Lucca and Pisa (7 yr old cannot climb) , again, you can DIY (a bit trickier--go to Lucca 1st and work your way back). In Lucca, walk or bike on top of the wall (like a park) and climb the towers. Use one of the France days as a 1/2 day walk around and beach day. If your kids are not too tired, take public bus/train to a medieval hill town.
some ideas for N Europe with kids:
Stockholm: take 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 you 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.
St P--it's a LONG day even for adults. Book a private tour, and be prepared to let your kids rest the next day. When we went StP was one long day. If 2 days, I might let my kids stay in the kids club the 2nd day.
Although many say a cruise is not the best way to see Europe, with kids, we found it an excellent way--esp with comfy room, pool, and lots of food they recognized and would eat. Esp. in Med, we usually came back, had a quick snack, had a swim and then dinner and a show or just veg and watch some TV--great mix for us. We have done land also and my kids all prefer a cruise.