Did you already check the weather data on the CM's page?
Click where it says 'Weather'. You'll find forecast, current weather and climate per month.
Also, lots of useful info there. I believe it's perfect for sailing (close to the village and the chief is a long-standing sailing resp. of CM). I'd say in August, you'll have tons of families there.
As you should know, Palmyre is on the Atlantic Ocean and it won't be as hot as on the Mediterranean Sea.http://www.clubmed.co.uk/cm/resorts-europe-france-la-palmyre-atlantique_p-341-l-EN-v-LPAC-ac-vh.html