Hi Gill,
We are going to Royan in July, having been to La Rochelle 2 years ago. Both times we have used le Mans, as a stop over and would not consider doing the whole journey in one go. Even though our son is now 11 and well used to long journeys he would find it too far to go, never mind the rest of the family. We learned many years ago when touring abroad not to try to go too far in a day. We personally never consider much over 300 miles, so we split 400 into 2 journeys. When we have tried longer journeys it takes to much time for the driver to recover!
It's all down to personal opinion and experiences, these are ours!
Enjoy your holiday
Alan