We recently did our first summer day trip to Ile D'Oleron. Not terribly like its sister island Ile de Re; it takes a much more natural, laid back approach to life. Long sweeping beaches, a port and a chateau (from what I have seen so far). There is also an abundance of places to delight in oysters, mussels and other seafood.
Coming back to the children.........the island sports great waves. Ideal for body boarding and surfing. The first beach we came to seemed to be more of a surfing beach for adults, with waves probably around 10 ft high. Not as wild as the coast savage, but probably ample size for the average surfer. Go along the coast a short drive and you find places that are more compact and appropriate for kids and adults alike. There are kids body boarding and playing on large rubber rings on the waves as well as surfing. You can book a surfing lesson whilst you are there too. Be aware that there do not appear to always be life guards on duty.
If surfing is not your thing, we have found three places where there are "treetop adventures" or parcour adventures as they are called here. One is at St George de Didonne called Parcour Adventures - it is compact so those parents not participating can easily keep an eye on the children. There are different levels of challenge and fantastic sea views. The second is their sister company at La Palmyra, which is 5 minutes from the tourist information office. Whilst we haven't been on this one, it looks equally as good but without the sea views. The last one, and my children's favourite, is at Royan and is called Coolongalook Park Adventure. Each element is more hidden within the trees. A word of warning though - perhaps wear long trousers as I got a few mosquito bites whilst there. If you want to take a picnic there is a small area with tables and seats at the start but no refreshments are available on site. You can expect to pay around Euros 15 per child.