Arcachon has a lot going for it but it’s almost a suburb of Bordeaux. Don’t plan on using the car much unless you wish to spend your holiday sitting in traffic jams. It won’t be cheap.
The lakes along the coast are warmer and safer for kids when the Atlantic gets angry. A mobile home on Les Ormes is a step up from camping. We hired one once and were envious of the campers because there’s no air-con and tents cool faster at night. The next time we camped. A lake, the ocean via a cycle path (hire bikes), the Medoc vineyards by bike or car, Bordeaux for a day trip, Royan by the BAC. There’s my suggestion.
As for the Med, the sea west of Montpellier is often no warmer than on the Atlantic coast, around 20°C thanks to an off-shore breeze blowing away warm surface water and bringing up colder water. However, east of Cassis it’s warmer and around St Tropez often over 26°C in August. Out of season I like the area but between the 21 of July and 21 of August it’s very busy. If you like crowds and all the noise, pollution and traffic that comes with them, fine.
Northern Spain can be a bit humid and the sea rarely gets much above 20°C, but on the plus side it never gets too hot and it’s not as manic as the Med. Comillas, Castro Urdiales and Zarautz are pleasant resorts near the towns of Santander, Bilbao and Donista.