for our anniversary my wife and I stayed in Brewster (not far from Dennis) at sweetwater farm campground. The campground was Awsome, campsites were seperated by brush and trees enough to give some privacy, there is a playground for every bathroom, a lake for boating (no swimming (MA law requires a full time life guard for every pool or beach)) the bathrooms are very clean, and the great Cape Cod rail trail is just across the street. BUT, the nearest beach is a bike ride or drive away( too far to walk, even for my 3 mile a day wife) and a parking pass is $15/day. The $15/day seems to be standard for every beach on the mid or upper cape. Brewster is also on the cape cod bay side, so no surf (my kids love boogie boarding) and at low tide the water receeds over a mile from the high tide beach.
The camp map where you stayed, shows two playgrounds, but for my TV-less kids, a beach will keep them occupied until they are ready to drop every day.
Until this coming summer we have stayed for free on York Beach. (I hate to say it but you missed the best lighthouse in ME, the Nubble in York, ME) Now we are looking to go camping for vacation, as my parents cabin there is going on the market. The cape is the nearest warm water beaches for us, even in August we wear wet suits to play in the water and some days to play on the beach in York.