I have 3 sons (10,13,15) and these are some of the things they'd be interested in doing.
From Barcelona, head to Carcassonne to get a taste of a medieval castle and fort, then head to the Lascaux caves and vicinity. Take train directly to Paris. Something like days 6 to 8 in Rick's South of France tour reversed(as listed below).
Since you're from the LA area, Disneyland Paris may not be of interest but it does appeal to many kids as a one day diversion from typical Paris sightseeing. Also, check out Vaux le Vicomte for a quick trip outside Paris.
From Barcelona, you could also fly/take train directly to Paris, spend a few days there and then take train to London. Paris and London have a lot to offer for kids and many flight options.
I also really like Rome, Venice and Switzerland as kid-friendly destinations but it's harder to find non-stop or inexpensive flights from those locations.
If you haven't already, check with your cruise line for airfares. They often have much better one way airfares than available elsewhere for people taking transatlantic cruises.
ITINERARY SOUTH OF FRANCE-
This morning we'll drive south, crossing the scenic valleys of the Lot and Tarn Rivers as we make our way to Europe's largest fortified city: delightfully medieval Carcassonne. After a walking tour of Carcassonne's fortified old town — La Cité — you'll be free to savor views from the stony ramparts, wander through the cobbled lanes, and maybe sample a steamy bowl of cassoulet with a glass of vin rouge.
Market Day in Sarlat
Take a vacation from your vacation! You'll have the entire day to savor the colorful weekly market in one of France's most pedestrian-friendly towns. Rub shoulders with local shoppers, sample foie gras and salivate over the famous black truffle as you shop for your fantasy picnic lunch. Today you can make it a tasty reality! Then stretch your legs with a countryside walk. No bus. Walking: light to strenuous (your choice).
Cave Art and Canoeing
Today begins with a chance to appreciate some of Europe's oldest art: the 13,000-year-old drawings of Rouffignac Cave. We'll hop aboard a subterranean train to marvel at engravings and drawings depicting bison, horses, woolly rhinoceros, and…mammoths. There are so many mammoths depicted locals have nicknamed it "the cave of 100 mammoths." This afternoon (weather permitting), we'll canoe down the Dordogne River from Cénac to Beynac, gliding past medieval castles and villages. If the weather is hot, we may even stop for a swim along the way. Bus: 3 hrs. Walking: strenuous.