Having benefitted from MNers wisdom regarding the Vendee, I thought I would come back and post a review. We went in late June/early July and the party comprised me, my husband and our son, 2.10.
What we enjoyed:
Mervent Zoo - lovely day out with beautifully landscaped grounds, quite a few animals roaming free and, for the most part, large enclosures. Cafe food indifferent, but there is a picnic area.
Green Venice - we enjoyed an hour's boat trip, even with a toddler!
Aquarium of the Seventh Continent (?), Talmont St Hilaire - lovely, huge tanks and quite quiet. Our toddler loved it!
Tranche sur Mer - loved the beach slightly to the south of the main part of town.
What didn't work for us:
Long driving distances - it seemed to take a long time to get anywhere. Everything was very spread out. We were inland so were anticipating a bit of a drive to the coast, but it still took us ages to get to La Rochelle or Mervent. More importantly, the time in the car threw out our son's sleep pattern so we lost our evenings!
Flat landscape - don't go there if you love hills!
General character - we struggled to find the French town/village life we were hoping for and would have enjoyed. Hardly any of the towns/ villages seemed to have shops or cafes, we saw quite a few shuttered-up houses and there seemed to be lots of big retail box developments along the major roads.
Hope this is helpful to someone.