Driving from Barcelona to French Riviera with kids

Driving is about 8-9 hours. The problem is that returning a car in France that you've rented in Spain will incur large fees. I know that Via Michelin says almost 7, but add getting out of the city, toilet breaks, traffic jams.

The train is 9 hours. You have to change from one high-speed train to another in Nimes, and then change again but to a slow, local train in Marseille. The train time is less than 4 hours from Barcelona to Nimes, but with changes and the requirement that trains go slowly on the Marseille to Nice tracks, this drags out the total trip to 9 hours. Granted the very last part along the water is beautiful.

Here's what I'd do: I'd take the train from Barcelona to Montpellier, Gare St Roch which is only three hours, or on to Nîmes Gare de Centre ville, which is 3.5 hours. Do not book trains to the TGV stations because you won't be in the center of these cities. Both city stations receive TGV trains, too. I'd spend the night, visiting either the Medieval historical center of Montpellier (my preference) or seeing the Roman arena and Maison Carrée in Nîmes. Then I'd rent a car from one of these two cities and drive. The drive is less than four hours. The train is 5-6.

That way you avoid a drop charge, you can take the faster option for each end of the trip.

Are you aware of the foreign rental car drop off fees? They can get quite expensive in 100s+ of euros.

Will you be spending some time beforehand in Barcelona? The city itself needs 4 nights minimum to see.

What time of year are you going?

I wasn't aware of the fees for dropping off the rental car in another country, so we will definitely avoid that! We have been to Barcelona before, we will be going back there to visit friends. So we will plan to spend 3-4 days there, and then move on to the French Riviera. We will be traveling in June 2023.
i'm just trying to figure out the best way for us to travel, since I will be with my daughter, who is 10. It sounds like it might be best to take the train into France, and then rent a car. We would like to visit towns along the Riviera, and then fly out of Nice.
Or, we could go the opposite way, and end our trip in Barcelona.

And, I'm wondering what towns on the French Riviera are "must sees" with kids.

Honestly I think the French Riviera is overhyped and overpriced.

Have you seen the charming Costa Brava or ventured into the Pyrenees foothills of northern Catalonia? If it were my holiday I just stay on the Spanish side, rent a car and see the sites of northern Catalonia. Then return the car back in Barcelona and fly out from there. That way you have the flexibility of a car without the expensive foreign drop off fee.

There are plenty of fun activities (like snorkeling, sea kayaking) and excellent beaches along the Costa Brava. When I was a young boy my family would spend the summer holidays at Tossa de Mar. Alternatively Calella de Palafrugell offers a more low key beach destination. One can hike between the Costa Brava towns along the coastal Camino de Ronda.

Inland in the Pyrenees foothills there are plenty of charming medieval towns and villages that are popular with local Catalan families on their summer holidays. Rupit, Besalú, and Castellfollit de la Roca come to mind, with good hiking in the nearby Vall de Núria. Promise you will spend half the money but have twice the amount of fun than if you went to places like Saint-Tropez or Cannes on the French Riviera.

Cross into France by train and pick up rental car. Take a look at Collioure, France not too far north of Spain’s Costa Brava. A charming waterfront town, chances to swim, beautiful beach.

Wholeheartedly agree with Suki (apart from picking up a rental car).
Collioure, and nearby Port Vendres, are excellent choices for a chilled and child-friendly stay. And, it is possible to travel by train to Collioure from Barcelona for only 10,10€ (normal adult fare). Or, take the AVE or AVANT high-speed train from Barcelona to Girona (for around 9-17€) then take the regional train to Portbou (around 6€) and then Cerbere and Collioure (1€!). Know so because I did this trip on November 4th last year and my sister-in-law from Cape Cod just traveled up to Collioure from Barcelona yesterday (Jan.2). She messaged me a couple of hours after she arrived: 'I love this place and totally get why you two love it too.'
The reason for our trip back in November was to visit Antonio Machado's grave, and also to celebrate my birthday. We thoroughly enjoyed our all too brief sojourn there. We'll be back there soon.
Check this out: https://www.ricksteves.com/watch-read-listen/read/articles/charming-collioure
If you plan the rail connections right you can do the trip from Barcelona to Collioure in two and a half hours (much less time than it would take you to drive — even with perfect traffic flow). Dump the hassle and expense of hiring a car — have the train take the strain.
Enjoy your visit.

There are also nice coastal towns heading west from Barcelona like Sitges and Tarragona. We liked our Tarragona day trip so much in October 2022 we are going back for two nights in February 2023. The Roman ruins should be enough to interest a 10 year old and it only takes about 90 minutes by train to get there from Barcelona. Your daughter would probably also like Monserrat, especially the cable car or rack railway.

AussieNomad makes a great point! Just south of Barcelona is the Costa Dorada, a land full of ancient olive trees, citrus groves, sandy beaches, lazy beach towns, time-warped castles, and Roman Ruins. You can visit the Roman ruins of Tarragona, or the beach town of Sitges, or PortAventura one of the largest theme parks in Europe.

And, I'm wondering what towns on the French Riviera are "must sees" with kids.

None.

I also agree that it is overhyped and overpriced, I genuinely don't see the attraction.

I took my kids to Nice, they were very bored and so was I. We found pottering around the villages outside of Nice during the morning, stopping for lunch and then returning to the villa for pool time was much better.

Thank you to everyone for your helpful responses! This will definitely guide me as I make plans for our trip. Much appreciated!