Delaware Seashore National Park. Full hookups, 2 minute walk over the dunes to the beach. Its right on the Indian River Inlet as well. Recently renovated (south side of the inlet) and brand new (north side of the inlet) spots. Clean bathrooms, level sites. Price is pretty good.
Its about 30 minutes north of Ocean City, but about 10 minutes south of Rehoboth Beach DE and Dewey Beach DE and 5 minutes north of Bethany Beach DE.
Bethany and Rehoboth are great family friendly beaches with boardwalks, plenty of food and shops.