Take a look around Fontenay le Comte in the south Vendee. The south is much nicer than the North. Beaches are about 40 mins away, so is La Rochelle. Nearby Mervent forest is lovely. Green venise is quite nice and the fabulous and amazing Puy de Fou is under an hour.
If you don't mind travelling a bit further south of La Rochelle, the area around La Palmyre is popular with families. The sand is beautiful and the coastline is a bit more exciting than the Vendee. Great cycle paths through the forests and lots of campsites and small hotels. There is a also a Club Med type of place too. La Palmyre Zoo is fantastic. I would try and stay near the coast as the roads in summer are very busy. Cycling is very popular for that reason. There are lots of interesting places to visit in the area. Our favourite little town is St Palais Sur Mer which has lovely beach restaurants and ice cream parlours.