Futuroscope is next to the Autoroute so the trip is not to bad, if you do not stay for the evening laser showm on the lake you will have wasted your journey and not seen the BEST part of the day.
The trip is OK but you and the kids will love the evening show and will not get back to Royan until after the sites gates are locked. Bon Anse Plage has plenty of parking outside and site security keep an eye on parked cars!
Have been to uturoscope a few times as the shows vary and it is one of the best days out you can pay for in France. My kids always prefered it to Disney