One place you might consider is the northern Outer Banks. We rented a beach house for a week there when my kids were about that age. They absolutely loved it. If you stay in Carova you will have to have a 4WD vehicle but the benefit is that the place is not crowded at all. You will have plenty of space at the beach. The houses are huge and have everything you could need - full kitchens, several bedrooms, barbecue grills, hot tubs, fishing gear, wifi, satellite, kayaks, etc. There are wild horses roaming the island (my kids' favorite part of the trip) and you can always fight your way through traffic down to the rest of OBX if you get bored.