Get the book, 'English Family holidays in the Vendee' by somebody Bird (available on Amazon), as it is great!
We had our best ever holiday there last year, and we are going back this year.
The beaches are great, flat, clean and gently shelving. Many are backed by sand dunes and/or pine forests, where we would picnic. Didn't go to the water parc in The Ile de Noitmoutier, but it looked excellent from the outside! Would go there if we went back to St Jean. Well worth going to the Ile via the causeway, which is only open when the tide is out....crazy feeling but memorable!
It was very hot when we went there, so we tended to go to the beach early, shelter from the sun (forset walks etc) when it was hot and then sit in the pool on the site in the afternoon. We did go to Puy de Fou (sp?) which was a long drive (about 1.5 hours) but it is amazing . A french historical theme park, beuatiful grounds, with piped classical music! And the most amazing historical 'reconstuction show, including a gladiatorial fight, charriot races, christians being thrown to the lions (not kidding!) and castles being stormed with knightes in shining armour. All done in french but we got by! Kiddies shows, mini train round the grounds.....excellent stuff.
One thing to bear in mind is the heat/sun, so make sure you all take hats and stuff....it is a lot hotter than Britany.
If you are there in July or Aug it is worth going to Challens where they do out the whole town in 1900 clothes, vintage cars, argiculture, bike races, old fire engines and give demos of historical practices (every other weekend). And it is all free! The kids get to hold ducklings etc. Big benches outside the Hotel de Ville where you can buy local food and wine and eat on long benches, sitting and chatting to the locals...who love it if you give your school French a shot!
So much to do, naff little fair grounds that the kids love, wine, food....god I can't wait til the summer.