You've picked a fine area of France and of LR to visit. There are so many things to see. I highly recommend the Michelin Green Guide to LR which is an invaluable resource for things to do, driving routes and advice on restaurants. We found the Michelin Restaurant app useful, too.
We spent a month just outside Pezenas one summer. It was one of our best holidays. We alternated lazy days beside the pool and interesting day trips where the choices are plentiful. We didn't have young children along, however the things I'll mention will likely have some appeal to the entire family. All are within a couple of hours' drive from Roquessels. Some things have already been mentioned.
First, the beach. We liked Marseillan Plage a lot. It's a fine beach that wasn't crowded during June, but of course July might be different. The town of Marseillan, a short distance away, has several decent restaurants around the old harbor.
The Millau Viaduc is spectacular. I think even the girls will be impressed. The visitor center is interesting and the walk up to the viewpoint from there isn't difficult. Roquefort-sur-Soulzon is near Millau, but I couldn't get any of my group interested in a walk through the cheese caves. Saint-Guilhem-le-Désert as mentioned by Alex is lovely and could fit into the day as well.
The village of Minerve is fascinating. The history of the Cathars is virtually unknown in America as it may be in the rest of Europe as well. The setting is striking with fine views. A couple of good restaurants and some good shops inhabit the ancient place. On the way to Minerve the Canal du Midi town of Capestang has a pleasant "place" with restaurants, etc. Near the church is a plaque honoring the local men who were rounded up by the Nazis. The whole area was a center of the Resistance movement in WWII.
Aigues Mortes and the Camargue fit together nicely for a day trip. AM is completely authentic unlike Carcassonne with its somewhat controversial restoration work. The Michelin Restaurant app came in handy in AM by leading us to a very good quiet restaurant away from the touristy spots. The Camargue is a unique part of France on a flat plain with flamingoes, white horses and black bulls. Saintes-Maries-de-la-Mer has an interesting old church and a fine sandy beach.
Collioure is well worth the drive but be warned that parking is a challenge. If you arrive early enough the central parking area may have spots. The so-called Cathar castles might fit into the day. It isn't necessary to make long hikes to appreciate the rugged beauty of the castles.
I have to disagree about Beziers. The city has a charm and interest. The Jardin des Poets is a fine park. Nearby the Allees Paul Riquet is lined with restaurants and shops. A highlight is the cathedral which sits on the site of the massacre that took place during the sack of the city in the Albiginsian Crusade. There are fine views over the countryside and the Canal du Midi. The Fonserranes Locks are interesting and fun to visit. There's a path beside the canal for walking and biking.
Roquebrun is beautiful. Le Petit Nice restaurant has good food and great views over the river and valley. The Saint Chinian wine appellation nearby produces great wines. Take time to visit some wineries. The children will be welcome.