Saltwater Farm in Thomaston!!! Okay, it's technicallt in Cushing, but just over the line. Great place we go to every year. Probably wouldn't excite a kid, though there is a pool. It's primarily an adult CG.
That said, however, I'd be surprised if you found ANY open space this close to the lobster festival. Maybe for next year.
We go to Camden every year and Rockland is close by. I would look for a site at Camden Hill State Park. You can get W/E and can make your reservation online. You will appreciate the little town of Camden with all those little boutiques, antiques, and restaurants with a beautiful view of the Atlantic.