August along this coast is choc-a-bloc, well probably not as bad as cote d'azur but busy all the same. We stayed in St Jean de Luz last year (outside the town, on the coast) and found the 5 min drive into the town took about 30 mins and parking was impossible. the restaurants were full etc. The short drive down into Spain took twice as long as it should have etc etc.So, bear that in mind.
The whole coast from Bordeaux to Biarritz and south of there is fabulous, it's one long stretch of sandy beach. The sea is quite rough though (great for surfing) so children probably prefer the pools. We've stayed in Vieux Boucau, Messanges, and Arcachon as well and absolutely love the whole area. Hendaye is supposed to be a good spot too.
In general I would say the busiest areas are Biarritz and southwards to Spanish coast and the first few resorts north of Biarritz are busy too. Arcachon, just south of Bordeaux is very busy too. The quieter areas are probably between these 2 including St Girons, Leon, Vieux Boucau.
Enjoy, it's a fabulous area!