Spent two weeks on the Cape last summer and were happy with both places we stayed.
This one is full service with a fantastic pool - especially for kids:
Cape Cod Campresort & Cabins
176 Thomas B. Landers Road
East Falmouth, MA 02536
Tel: (508) 548-1458
camp@capecampresort.com
http://capecampresort.com/
This one is basic and an easy drive to Provincetown or the local beaches:
Atlantic Oaks Campground
3700 State Highway 6
Eastham, MA 02642
Tel: (508) 255-1437
Fax: (508) 247-8216
atlantic_oaks@capecamping.com
http://atlanticoaks.com/
They both sell out, so if you're planning to go this summer, I'd make reservations ASAP!