Barcelona to Carcassonne

Carcassonne is at it's absolute worst (very, very crowded) during the exact hours that you would be there. Not worth it, IMHO.

If you absolutely must do Carcassonne on this trip, then go the day before and spend the night - being there late and early, and avoiding mid-day is a better strategy anyway. But it's a long way to go out of your way.

Consider an alternative: if you just want to show off a huge medieval fortified city with impressive walls and battlements, consider Avila as a (perhaps) more conveniently-located substitute. That depends on your itinerary for Spain, but Avila is not terribly far from other major attractions around Madrid. Other than the very impressive walls, there's not much about Avila that justifies a long detour - but that's equally true of Carcassonne. If you can hit Avila with less of a crazy detour, it'll cross off "Impressive Walled Medieval City" fro your son's list. After visiting Avila, every time you see a movie supposedly set in the middle ages, the castle walls will look familiar.

Or Girona, which has a walkable wall and many other virtues and is 38 minutes from Barcelona via the fast train.

I haven't tried Barcelona to Carcassonne and back in a day, but I agree with considering potential alternatives.

Girona, which Acraven has brought up, is an ancient city in northern Catalonia, accessible via the AVE high-speed train. It has an impressive Cathedral, towering medieval walls, narrow winding streets, a Moorish bath complex, and one of the best preserved Jewish Quarters in Europe. The Onyar river snakes it's way through Girona, giving the town a Florence-esque feel.

Another alternative could be Peñíscola, which is a historic and beautiful town about 2:30 hr south of Barcelona via car. With its cobbled streets and whitewashed houses, the old town of Peñíscola sits on a big rock that juts out into the sea. The town is dominated by a large Knights Templar castle that looks like something out of the Crusades. It was later home to 15th century Antipope Benedict XIII. Recently, Peñíscola was used as a filming location in the T.V. series Game of Thrones.

Thank you everyone! Yes, it seems too complicated. I think we might stick to Roman sites instead and spend the day in Tarragona. I never thought of Girona. That sounds great.
We might stop in Avila in our way to Salamanca but if I remember well from my my Europe backpacking trip 27 years ago, really the walls are all to see. Altgough I was there for the procession of Epiphany Day ( Día de Reyes) and I went there because because of a great Spanish miniseries of Saint Teresa of Avila that I totally loved! That is why stopped there, to see her finger! Alex couldn’t care less about that part. But Tarragona and Scipio Africanus war against Cartago might do the “trick”

I have sort of made it my life's work to keep people from spending time on a trip to Avila when there are so many more interesting places to see. Thank you for your support!

Given your son's age, I highly recommend a trip to Carcassone! It will be an incredible experience for him. Especially if you get the amazing board game of the same name, and play that together before you go. And do the overnight, so that you can see the medieval walls all lit up. Fabulous idea. My teenage boys loved our visit and it's a favorite family memory. We still do board game nights together (when they come home from college) and still toast to the memory of the evening there. All the challenges of the hassle were easily forgotten, only the great memory remains, as part of the cement of our family. Cheers!